External Drives Sleep Guard (macOS)

Stop the dreaded “Disk Not Ejected Properly” when you close your Mac’s lid on battery with external drives attached. On AC power, drives stay mounted so long jobs (LLMs, copies, servers) keep running.

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What it does

• Clean unmount + eject (whole disk) right before battery sleep
• Auto‑remount by Volume UUID after wake (with a retry window)
• Targets Samsung T7 only by default; can switch to all removable or specific serials
• Works on Apple Silicon and Intel

It uses the tiny background agent SleepWatcher to run two user hooks: ~/.sleep (before sleep) and ~/.wakeup (after wake).

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Quick install (one command)

This one‑liner installs SleepWatcher, writes the helper at ~/.local/bin/sleep-guard.sh, creates the hooks, starts the service, and runs a self‑test. Default mode: Samsung T7 only. You can change it later.

if ! command -v brew >/dev/null 2>&1; then echo "Install Homebrew first: https://brew.sh"; exit 1; fi
brew list sleepwatcher >/dev/null 2>&1 || brew install sleepwatcher
mkdir -p ~/.local/bin ~/.cache/extdisk-park
cat > ~/.local/bin/sleep-guard.sh <<'SH'
#!/usr/bin/env zsh
# External Drives Sleep Guard — eject before battery sleep, remount on wake.
set -euo pipefail
log="$HOME/Library/Logs/sleep-guard.log"      # event log
state="$HOME/.cache/extdisk-park/uuids"       # cached Volume UUIDs for remount
# ---- Config knobs (edit later if needed) ----
HOLD_SECONDS=12      # hold system awake this long before sleep (finish unmount/eject)
WAKE_RETRY=30        # seconds to keep retrying remount on wake
MODE="t7"            # "t7" (Samsung T7 only) | "all" (all external removable)
SERIALS=()           # optional: ("SERIAL1" "SERIAL2") to target specific devices
# ---------------------------------------------
_ts(){ date "+%Y-%m-%d %H:%M:%S"; }
_onbat(){ pmset -g ps | grep -q "Battery Power"; }
# Discover WHOLE disks to manage (e.g., /dev/disk6 /dev/disk7)
_targets(){
  /usr/sbin/diskutil list external | /usr/bin/awk '/^\/dev\/disk[0-9]+ \(external/{print $1}' | while read -r dev; do
    p=$(/usr/sbin/diskutil info -plist "$dev") || continue
    name=$(echo "$p" | /usr/bin/plutil -extract MediaName   raw -o - - 2>/dev/null || :)
    proto=$(echo "$p" | /usr/bin/plutil -extract BusProtocol raw -o - - 2>/dev/null || :)
    ser=$(  echo "$p" | /usr/bin/plutil -extract SerialNumber raw -o - - 2>/dev/null || :)
    [[ "$MODE" = "t7" && ( "$name" != *T7* || "$proto" != *USB* ) ]] && continue
    if (( ${#SERIALS[@]} )); then
      keep=false; for s in "${SERIALS[@]}"; do [[ "$ser" == *"$s"* ]] && keep=true; done; $keep || continue
    fi
    echo "$dev"
  done
}
# Cache Volume UUIDs that live on those targets (so we can remount by UUID later)
_cache(){
  tmp="$state.tmp"; :>"$tmp"
  local t; t=($(_targets))
  for v in /Volumes/*; do
    info=$(/usr/sbin/diskutil info "$v" 2>/dev/null || :); [[ -z "$info" ]] && continue
    [[ "$(echo "$info" | /usr/bin/awk -F': *' '/Device Location/{print $2}')" != External ]] && continue
    uuid=$(echo "$info" | /usr/bin/awk -F': *' '/Volume UUID/{print $2}')
    leaf=$(echo "$info" | /usr/bin/awk -F': *' '/Device Node/{print $2}')
    [[ -z "$uuid" || -z "$leaf" ]] && continue
    whole=$(/usr/sbin/diskutil info -plist "$leaf" | /usr/bin/plutil -extract ParentWholeDisk raw -o - - 2>/dev/null || :)
    [[ -z "$whole" ]] && whole=$(echo "$leaf" | /usr/bin/sed -E 's#(disk[0-9]+)s[0-9]+#\1#')
    for d in "${t[@]}"; do [[ "/dev/$whole" == "$d" ]] && echo "$uuid" >>"$tmp"; done
  done
  if [[ -s "$tmp" ]]; then
    sort -u "$tmp" -o "$state"
    echo "$(_ts) register: $(tr '\n' ' ' <"$state")" >>"$log"
  else
    echo "$(_ts) register: no targets; keep previous" >>"$log"
  fi
  rm -f "$tmp"
}
_uuids(){ [[ -s "$state" ]] || _cache; cat "$state"; }
_do_sleep(){
  if ! _onbat; then echo "$(_ts) sleep: AC -> leave mounted" >>"$log"; return 0; fi
  echo "$(_ts) sleep: battery -> unmount+eject" >>"$log"
  /usr/bin/caffeinate -disu -t "$HOLD_SECONDS" &>/dev/null &  # hold sleep briefly
  /bin/sync                                                   # flush writes
  for dev in $(_targets); do
    /usr/sbin/diskutil unmountDisk force "$dev" &>/dev/null || true
    /usr/sbin/diskutil eject "$dev"            &>/dev/null || true
    echo "$(_ts) sleep: processed $dev" >>"$log"
  done
}
_do_wake(){
  echo "$(_ts) wake: remount" >>"$log"
  /bin/sleep 2
  for u in $(_uuids); do
    for i in $(/usr/bin/jot - 1 "$WAKE_RETRY"); do
      /usr/sbin/diskutil mount "volumeUUID:$u" &>/dev/null && { echo "$(_ts) wake: mounted $u" >>"$log"; break; }
      /bin/sleep 1
    done
  done
}
case "${1:-}" in
  register) _cache ;;
  sleep)    _do_sleep ;;
  wake)     _do_wake ;;
  test)     _cache; _do_sleep; /bin/sleep 3; _do_wake ;;
  *) echo "usage: $0 {register|sleep|wake|test}"; exit 1 ;;
esac
SH
chmod +x ~/.local/bin/sleep-guard.sh
printf '%s\n' '$HOME/.local/bin/sleep-guard.sh sleep' > ~/.sleep
printf '%s\n' '$HOME/.local/bin/sleep-guard.sh wake'  > ~/.wakeup
chmod +x ~/.sleep ~/.wakeup
brew services restart sleepwatcher
~/.local/bin/sleep-guard.sh test
echo "Log: $HOME/Library/Logs/sleep-guard.log"

Check the log:

tail -n 60 "$HOME/Library/Logs/sleep-guard.log"

You should see sleep: processed /dev/disk… followed by wake: mounted ….

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Configure what gets managed

Option A — All removable drives

Switch from T7‑only to all external removable disks.

nano ~/.local/bin/sleep-guard.sh   # or your editor
# set: MODE="all"
brew services restart sleepwatcher
~/.local/bin/sleep-guard.sh register

Option B — Specific devices by serial

Pin the behavior to one or more device serials.

# Find serials for external disks (not all bridges expose one)
for d in $(diskutil list external | awk '/^\/dev\//{print $1}'); do
  echo "== $d =="; diskutil info -plist "$d" | \
    plutil -extract SerialNumber raw -o - - 2>/dev/null || echo "(no serial field)"
done

Then set inside the script and restart:

SERIALS=("S6QJNA0R123456" "S6QJNA0R654321")

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Verify it works

1. Unplug AC so you’re on battery
2. Close the lid for ~15–20 s
3. Reopen and check:

tail -n 120 "$HOME/Library/Logs/sleep-guard.log"

Expected: sleep: processed /dev/diskX then wake: mounted <UUID> and no macOS eject toast.

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Tuning & tips

• HOLD_SECONDS → increase if your Mac falls asleep too quickly (try 18)
• WAKE_RETRY → increase if USB devices re‑enumerate slowly after wake (try 45)
• Reduce background churn on target volumes:

sudo mdutil -i off /Volumes/<YourVolumeName>

• System Settings → Privacy & Security → Allow accessories to connect → Always
• Advanced (optional) if deep hibernate races the hook on battery:

sudo pmset -b hibernatemode 3
# revert later:
# sudo pmset -b hibernatemode 25

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Uninstall

Stops the agent and removes the helper + cache. Your disks/data are untouched.

brew services stop sleepwatcher
rm -f ~/.sleep ~/.wakeup ~/.local/bin/sleep-guard.sh ~/.cache/extdisk-park/uuids

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FAQ

Does this format or change my disks? No. It only unmounts/ejects and remounts.

Thunderbolt enclosures? Yes. Set MODE="all".

Do I need Homebrew? It’s the easiest way to install SleepWatcher. You can install SleepWatcher manually and keep the same script.

Where are logs? ~/Library/Logs/sleep-guard.log

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License

MIT — see LICENSE.