diff --git a/.github/workflows/nightly-harnesses.yml b/.github/workflows/nightly-harnesses.yml
index db6e527..3ff4fce 100644
--- a/.github/workflows/nightly-harnesses.yml
+++ b/.github/workflows/nightly-harnesses.yml
@@ -98,6 +98,20 @@ jobs:
           make -j"$(nproc)" kernel.elf
           file kernel.elf
 
+      # 51 harnesses copy disk.img to a scratch path and exit 2 without it. It
+      # is an untracked, gitignored 128 MB binary that had only ever been made
+      # by hand on one machine, so it simply did not exist on a runner: the
+      # first full nightly run lost most of the suite to "disk.img not found",
+      # which reads as a wall of kernel failures rather than a missing fixture.
+      #
+      # The fixture it copies in (userspace/info.elf.signed -- the 14144 bytes
+      # verify-fsyscalls.sh asserts on) is TRACKED, so this needs nothing from
+      # the build and the ordering here is not load-bearing.
+      - name: Build the FAT32 disk.img that the harnesses mount as drive C
+        run: |
+          bash tools/make-disk-img.sh
+          ls -l disk.img
+
       - name: Run the suite
         run: |
           bash verify/run-all.sh
diff --git a/tools/make-disk-img.sh b/tools/make-disk-img.sh
new file mode 100755
index 0000000..75b0c71
--- /dev/null
+++ b/tools/make-disk-img.sh
@@ -0,0 +1,98 @@
+#!/usr/bin/env bash
+#==============================================================================
+# make-disk-img.sh -- build the FAT32 `disk.img` the harness suite needs for C:.
+#
+# WHY THIS EXISTS
+#
+# 51 of the harnesses in verify/ reference disk.img, copying it to a scratch
+# path and exiting 2 if it is missing. Nothing in the repo built it: it was an
+# untracked, gitignored artifact that existed only on the machine where it had
+# once been made by hand. That is invisible locally -- the file is just there
+# -- and fatal anywhere else. The first full nightly run lost most of the suite
+# to "disk.img not found", which reads as a wall of kernel failures.
+#
+# It is NOT committed to git: a 128 MB binary in a repo, ~99.99% of it zeroes,
+# to hold one 14 KB file that is already tracked. Generating it takes under a
+# second; storing it does not.
+#
+# WHAT THE HARNESSES ACTUALLY REQUIRE
+#
+#   1. A REAL FAT32 volume. verify-ring3-fatls.sh checks the FAT32 signature at
+#      offset 0x52 and refuses a zeroed image, because against one C: would not
+#      mount and `ls C:/` would fail for a reason unrelated to what it tests --
+#      a vacuous run that scores as a kernel bug.
+#
+#   2. C:/HELLO.ELF, 14144 bytes. userspace/fileio.c opens it O_RDONLY and
+#      lseeks to SEEK_END; verify-fsyscalls.sh asserts the result is EXACTLY
+#      14144, so the size is load-bearing. It is never executed -- only read --
+#      so this is a size-and-content fixture, not a runnable binary.
+#
+#      We use info.elf.signed, which is 14144 bytes. Do NOT substitute
+#      hello.elf.signed just because the names match: it is 14388 bytes and
+#      would fail that assertion. (The hand-made image had a stale 14144-byte
+#      HELLO.ELF from an older signing run, which is why the name never
+#      matched its content.)
+#
+# Idempotent: rebuilds from scratch every time, so a corrupted image is fixed
+# by re-running rather than debugged.
+#==============================================================================
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+OUT="${1:-disk.img}"
+FIXTURE="userspace/info.elf.signed"
+FIXTURE_SIZE=14144
+
+command -v mformat >/dev/null 2>&1 || {
+    echo "ERROR: mformat not found (install mtools: brew install mtools / apt-get install mtools)" >&2
+    exit 1
+}
+
+# The fixture is tracked in git, so a checkout has it. If it is missing, say so
+# rather than silently producing an image without the one file C: must contain.
+if [ ! -f "$FIXTURE" ]; then
+    echo "ERROR: $FIXTURE not found (it is tracked -- is this a full checkout?)" >&2
+    exit 1
+fi
+
+# Assert the size the harness hardcodes. If signing ever changes it, fail HERE
+# with the real reason rather than 800 s later inside verify-fsyscalls.sh as a
+# confusing "SEEK_END reported N, expected 14144".
+actual=$(wc -c < "$FIXTURE" | tr -d ' ')
+if [ "$actual" -ne "$FIXTURE_SIZE" ]; then
+    echo "ERROR: $FIXTURE is $actual bytes, but verify-fsyscalls.sh asserts" >&2
+    echo "       C:/HELLO.ELF is exactly $FIXTURE_SIZE. Update both together." >&2
+    exit 1
+fi
+
+echo "==> Creating 128 MB FAT32 image: $OUT"
+rm -f "$OUT"
+# bs=1M, not 1m: GNU dd rejects the lowercase suffix outright, and BSD dd
+# accepts both. The suite used 1m and died on every Linux runner.
+dd if=/dev/zero of="$OUT" bs=1M count=128 status=none
+
+# Geometry matches the original hand-made image: 63 sectors/track, 16 heads,
+# 2 sectors/cluster, 32 reserved. -F forces FAT32 (mformat would otherwise
+# pick FAT16 at this size, and the driver under test is the FAT32 one).
+mformat -i "$OUT" -F -T 262144 -h 16 -s 63 -c 2 -R 32 ::
+
+mcopy -i "$OUT" "$FIXTURE" ::HELLO.ELF
+
+# INFO.ELF was also present in the hand-made image. No harness or userspace
+# source opens it -- the only two mentions in the tree are a usage example in
+# qemu_typist.py and a comment in verify-exec-frame-leak.sh warning NOT to use
+# it (it is a larger image and would move that harness's expected page count).
+# Included anyway so the generated image is a faithful replacement rather than
+# a subset, which keeps "it worked before" from meaning something different.
+mcopy -i "$OUT" "$FIXTURE" ::INFO.ELF
+
+# Verify what we produced rather than trusting mformat's exit code: check the
+# same FAT32 signature verify-ring3-fatls.sh checks, and read the file back.
+if ! dd if="$OUT" bs=1 skip=82 count=8 status=none | grep -q "FAT32"; then
+    echo "ERROR: produced image has no FAT32 signature at 0x52" >&2
+    exit 1
+fi
+back=$(mdir -i "$OUT" ::HELLO.ELF 2>/dev/null | grep -c "HELLO" || true)
+[ "$back" -ge 1 ] || { echo "ERROR: HELLO.ELF not readable back from $OUT" >&2; exit 1; }
+
+echo "==> OK: $OUT ($(wc -c < "$OUT" | tr -d ' ') bytes), C:/HELLO.ELF = $FIXTURE_SIZE bytes"
diff --git a/verify/auto-verify-exec.sh b/verify/auto-verify-exec.sh
index 63af7cd..8617e61 100755
--- a/verify/auto-verify-exec.sh
+++ b/verify/auto-verify-exec.sh
@@ -32,7 +32,7 @@ i686-elf-grub-mkrescue -o "$ISO" iso >/dev/null 2>&1
 echo "==> Fresh BLANK disk (forces first-boot password setup)"
 rm -f "$RUN_DISK" "$SERIAL" "$TRACE" "$MON_SOCK"
 # 128 MiB zeroed image, same geometry as disk.img
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU (monitor on $MON_SOCK)"
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
diff --git a/verify/verify-argv.sh b/verify/verify-argv.sh
index d3b1deb..fb00516 100755
--- a/verify/verify-argv.sh
+++ b/verify/verify-argv.sh
@@ -46,7 +46,7 @@ i686-elf-grub-mkrescue -o "$ISO" iso >/dev/null 2>&1
 
 echo "==> Fresh BLANK disk (forces first-boot password setup)"
 rm -f "$RUN_DISK" "$SERIAL" "$TRACE" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU (monitor on $MON_SOCK)"
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
diff --git a/verify/verify-bgjobs.sh b/verify/verify-bgjobs.sh
index 0e3b4da..11abafc 100755
--- a/verify/verify-bgjobs.sh
+++ b/verify/verify-bgjobs.sh
@@ -35,7 +35,7 @@ i686-elf-grub-mkrescue -o "$ISO" iso >/dev/null 2>&1
 
 echo "==> Fresh BLANK disk (forces first-boot password setup)"
 rm -f "$RUN_DISK" "$SERIAL" "$TRACE" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU (monitor on $MON_SOCK)"
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
diff --git a/verify/verify-chmod-owner.sh b/verify/verify-chmod-owner.sh
index 6296117..17feaa2 100755
--- a/verify/verify-chmod-owner.sh
+++ b/verify/verify-chmod-owner.sh
@@ -53,7 +53,7 @@ cp kernel.elf iso/boot/kernel.elf
 i686-elf-grub-mkrescue -o "$ISO" iso >/dev/null 2>&1 || { echo "FAIL: ISO"; exit 2; }
 
 rm -f "$RUN_DISK" "$SERIAL" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
     -boot d -m 256M \
diff --git a/verify/verify-ids-spray.sh b/verify/verify-ids-spray.sh
index 23a4cef..87d8586 100755
--- a/verify/verify-ids-spray.sh
+++ b/verify/verify-ids-spray.sh
@@ -120,7 +120,7 @@ run_case() {
     rm -f "$serial"
     mon=$(mktemp -u /tmp/tinyos-spray-mon.XXXXXX)
     disk=$(mktemp -u /tmp/tinyos-spray-disk.XXXXXX)
-    dd if=/dev/zero of="$disk" bs=1m count=16 >/dev/null 2>&1
+    dd if=/dev/zero of="$disk" bs=1M count=16 >/dev/null 2>&1
 
     qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
         -boot d -m 256M \
diff --git a/verify/verify-pipes-exec.sh b/verify/verify-pipes-exec.sh
index 3e72167..4d6d848 100755
--- a/verify/verify-pipes-exec.sh
+++ b/verify/verify-pipes-exec.sh
@@ -67,7 +67,7 @@ i686-elf-grub-mkrescue -o "$ISO" iso >/dev/null 2>&1
 
 echo "==> Fresh BLANK disk (forces first-boot password setup)"
 rm -f "$RUN_DISK" "$SERIAL" "$TRACE" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU (monitor on $MON_SOCK)"
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
diff --git a/verify/verify-pipes.sh b/verify/verify-pipes.sh
index b1aff78..3161528 100755
--- a/verify/verify-pipes.sh
+++ b/verify/verify-pipes.sh
@@ -46,7 +46,7 @@ i686-elf-grub-mkrescue -o "$ISO" iso >/dev/null 2>&1
 
 echo "==> Fresh BLANK disk (forces first-boot password setup)"
 rm -f "$RUN_DISK" "$SERIAL" "$TRACE" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU (monitor on $MON_SOCK)"
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
diff --git a/verify/verify-redirect.sh b/verify/verify-redirect.sh
index e1c21dc..eb53fd0 100755
--- a/verify/verify-redirect.sh
+++ b/verify/verify-redirect.sh
@@ -45,7 +45,7 @@ i686-elf-grub-mkrescue -o "$ISO" iso >/dev/null 2>&1
 
 echo "==> Fresh BLANK disk (forces first-boot password setup)"
 rm -f "$RUN_DISK" "$SERIAL" "$TRACE" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU (monitor on $MON_SOCK)"
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
diff --git a/verify/verify-ring3-fileops.sh b/verify/verify-ring3-fileops.sh
index cbb2c59..b0e5274 100755
--- a/verify/verify-ring3-fileops.sh
+++ b/verify/verify-ring3-fileops.sh
@@ -110,7 +110,7 @@ i686-elf-grub-mkrescue -o "$ISO" iso >/dev/null 2>&1 || { echo "FAIL: ISO"; exit
 
 echo "==> Fresh BLANK disk (forces first-boot password setup)"
 rm -f "$RUN_DISK" "$SERIAL" "$TRACE" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU"
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
diff --git a/verify/verify-sectest-redirect.sh b/verify/verify-sectest-redirect.sh
index 86eebf5..a78b3eb 100755
--- a/verify/verify-sectest-redirect.sh
+++ b/verify/verify-sectest-redirect.sh
@@ -132,7 +132,7 @@ echo "==> Guard: scheduler_stats() prints only outside its critical section"
 
 echo "==> Fresh BLANK disk (forces first-boot password setup)"
 rm -f "$RUN_DISK" "$SERIAL" "$TRACE" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU (monitor on $MON_SOCK)"
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
diff --git a/verify/verify-shell-history-logout.sh b/verify/verify-shell-history-logout.sh
index 5815549..8b73836 100755
--- a/verify/verify-shell-history-logout.sh
+++ b/verify/verify-shell-history-logout.sh
@@ -70,7 +70,7 @@ cp kernel.elf iso/boot/kernel.elf
 i686-elf-grub-mkrescue -o "$ISO" iso >/dev/null 2>&1 || guard_fail "mkrescue failed"
 
 rm -f "$RUN_DISK" "$SERIAL" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU..."
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
diff --git a/verify/verify-spawn.sh b/verify/verify-spawn.sh
index c9c668f..f3df185 100755
--- a/verify/verify-spawn.sh
+++ b/verify/verify-spawn.sh
@@ -47,7 +47,7 @@ i686-elf-grub-mkrescue -o "$ISO" iso >/dev/null 2>&1
 
 echo "==> Fresh BLANK disk (forces first-boot password setup)"
 rm -f "$RUN_DISK" "$SERIAL" "$TRACE" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU (monitor on $MON_SOCK)"
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
diff --git a/verify/verify-syscall-reject-counters.sh b/verify/verify-syscall-reject-counters.sh
index 03b8b0e..9f3c12e 100755
--- a/verify/verify-syscall-reject-counters.sh
+++ b/verify/verify-syscall-reject-counters.sh
@@ -85,7 +85,7 @@ cp kernel.elf iso/boot/kernel.elf
 i686-elf-grub-mkrescue -o "$ISO" iso >/dev/null 2>&1 || guard_fail "mkrescue failed"
 
 rm -f "$RUN_DISK" "$SERIAL" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU..."
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \
diff --git a/verify/verify-sysredirect.sh b/verify/verify-sysredirect.sh
index eba1fe3..b17bafe 100755
--- a/verify/verify-sysredirect.sh
+++ b/verify/verify-sysredirect.sh
@@ -105,7 +105,7 @@ echo "==> Guard: cmd_mem makes $mem_calls stream_printf call(s) in kernel.elf"
 
 echo "==> Fresh BLANK disk (forces first-boot password setup)"
 rm -f "$RUN_DISK" "$SERIAL" "$TRACE" "$MON_SOCK"
-dd if=/dev/zero of="$RUN_DISK" bs=1m count=128 status=none
+dd if=/dev/zero of="$RUN_DISK" bs=1M count=128 status=none
 
 echo "==> Launching headless QEMU (monitor on $MON_SOCK)"
 qemu-system-i386 -cpu Broadwell,+rdrand,+rdseed -cdrom "$ISO" \