diff --git a/build-windows-install-disk.sh b/build-windows-install-disk.sh
index deb48e6..6cbdee5 100755
--- a/build-windows-install-disk.sh
+++ b/build-windows-install-disk.sh
@@ -46,8 +46,8 @@ die() {
   exit 1
 }
 
-# The build has five steps; step prints a numbered heading for each of them.
-total_steps=5
+# The build has four steps; step prints a numbered heading for each of them.
+total_steps=4
 step_number=0
 step() {
   step_number=$((step_number + 1))
@@ -60,6 +60,18 @@ note() {
   printf '  %s\n' "$*" >&2
 }
 
+# show logs a command line before it runs; run logs it and then runs it. Use show
+# alone where the output of the command is captured.
+show() {
+  local line
+  printf -v line '%q ' "$@"
+  note "\$ ${line% }"
+}
+run() {
+  show "$@"
+  "$@"
+}
+
 os="$(uname -s)"
 case "$os" in
   Darwin|Linux) ;;
@@ -105,7 +117,11 @@ mkdir "$mount_dir"
 note "work directory: $work_dir"
 mount_method=""
 loop_device=""
+# Set while the output image is being written, so that a failed or cancelled
+# run does not leave a half-built image behind.
+partial_output=""
 cleanup() {
+  [[ -z "$partial_output" ]] || rm -f "$partial_output"
   case "$mount_method" in
     hdiutil) hdiutil detach -quiet "$mount_dir" >/dev/null 2>&1 || true ;;
     udisks)
@@ -127,6 +143,7 @@ mount_with_udisks() {
     return 1
   fi
   local setup
+  show udisksctl loop-setup -r -f "$iso"
   if ! setup="$(udisksctl loop-setup -r -f "$iso" 2>&1)"; then
     note "udisksctl loop-setup failed: $setup"
     return 1
@@ -142,8 +159,10 @@ mount_with_udisks() {
   # A desktop session may have mounted the loop device already, which makes
   # udisksctl mount fail with AlreadyMounted; use that mount point then.
   local mounted point
+  show lsblk -no MOUNTPOINT "$loop_device"
   point="$(lsblk -no MOUNTPOINT "$loop_device" 2>/dev/null | head -n 1)"
   if [[ -z "$point" ]]; then
+    show udisksctl mount -b "$loop_device" -t udf -o ro
     if mounted="$(udisksctl mount -b "$loop_device" -t udf -o ro 2>&1)"; then
       # Some udisks versions end the message with a period, others do not.
       point="$(sed -n 's/^Mounted .* at \(.*\)$/\1/p' <<<"$mounted")"
@@ -157,7 +176,7 @@ mount_with_udisks() {
   fi
   if [[ ! -d "$point" ]]; then
     note "no usable mount point for $loop_device (parsed mount point: '${point}')"
-    udisksctl loop-delete -b "$loop_device" >/dev/null 2>&1 || true
+    run udisksctl loop-delete -b "$loop_device" >/dev/null 2>&1 || true
     mount_method=""
     return 1
   fi
@@ -169,7 +188,7 @@ step "Reading the ISO $iso (it is UDF, so it has to be mounted)"
 note "ISO size: $(stat -L "${size_args[@]}" "$iso") bytes"
 if [[ "$os" == Darwin ]]; then
   note "mounting with hdiutil at $mount_dir"
-  hdiutil attach -readonly -nobrowse -mountpoint "$mount_dir" "$iso" >/dev/null
+  run hdiutil attach -readonly -nobrowse -mountpoint "$mount_dir" "$iso" >/dev/null
   mount_method=hdiutil
 else
   note "mounting with udisksctl"
@@ -187,7 +206,7 @@ note "install.wim: $(stat "${size_args[@]}" "$install_wim") bytes"
 step "Splitting install.wim into install.swm parts (FAT32 files must stay below 4 GiB)"
 split_dir="$work_dir/split"
 mkdir "$split_dir"
-wimlib-imagex split "$install_wim" "$split_dir/install.swm" 3800 >/dev/null
+run wimlib-imagex split "$install_wim" "$split_dir/install.swm" 3800 >/dev/null
 for part in "$split_dir"/install*.swm; do
   note "$(basename "$part"): $(stat "${size_args[@]}" "$part") bytes"
 done
@@ -204,12 +223,17 @@ image_mib=$(( (content_bytes + content_bytes / 20) / mib + 64 ))
 step "Creating a $image_mib MiB image with a GPT and one FAT32 partition"
 note "content to copy: $content_bytes bytes (without install.wim)"
 
-image="$work_dir/disk.img"
-truncate -s "${image_mib}m" "$image"
-sgdisk -o -n 1:2048:0 -t 1:0700 -c 1:WINPE "$image" >/dev/null
+image="$output"
+run mkdir -p "$(dirname -- "$output")"
+# --force replaces an existing image; truncate alone would keep its old content.
+run rm -f "$image"
+partial_output="$image"
+run truncate -s "${image_mib}m" "$image"
+run sgdisk -o -n 1:2048:0 -t 1:0700 -c 1:WINPE "$image" >/dev/null
+show sgdisk -i 1 "$image"
 sectors=$(sgdisk -i 1 "$image" | awk '/^Partition size:/ { print $3 }')
 [[ -n "$sectors" ]] || die "could not read the partition size"
-mkfs.fat -F 32 -n WINPE --offset 2048 "$image" $((sectors / 2)) >/dev/null
+run mkfs.fat -F 32 -n WINPE --offset 2048 "$image" $((sectors / 2)) >/dev/null
 
 # mtools addresses the partition by its byte offset inside the image.
 mtools_image="$image@@$((2048 * 512))"
@@ -222,6 +246,7 @@ copy_dir() { # copy_dir SOURCE_DIR TARGET_DIR_IN_IMAGE
     files+=("$entry")
   done < <(find "$source" -maxdepth 1 -type f ! -path "$install_wim" -print0)
   if ((${#files[@]} > 0)); then
+    note "\$ mcopy -Q -m -i $mtools_image <${#files[@]} files from ${source#"$mount_dir"} > ::$target/"
     mcopy -Q -m -i "$mtools_image" "${files[@]}" "::$target/"
   fi
 }
@@ -232,14 +257,12 @@ dir_count=0
 while IFS= read -r -d '' dir; do
   relative="${dir#"$mount_dir"/}"
   dir_count=$((dir_count + 1))
-  mmd -i "$mtools_image" "::$relative"
+  run mmd -i "$mtools_image" "::$relative"
   copy_dir "$dir" "$relative"
 done < <(find "$mount_dir" -mindepth 1 -type d -print0 | sort -z)
 note "copied $dir_count directories"
 note "copying the install*.swm parts to sources/"
-mcopy -Q -m -i "$mtools_image" "$split_dir"/install*.swm "::sources/"
+run mcopy -Q -m -i "$mtools_image" "$split_dir"/install*.swm "::sources/"
 
-step "Moving the image to $output"
-mkdir -p "$(dirname -- "$output")"
-mv -f "$image" "$output"
+partial_output=""
 printf 'Built Windows 11 installation disk: %s\n' "$output"