How do I eject the Time Machine backup drive automatically after each backup?

~/bin/timemachine:

#!/bin/bash

d="Time Machine"  # (change this to match the name of your backup drive)
diskutil mount "$d" && tmutil startbackup -b && diskutil eject "$d"

~/Library/LaunchAgents/timemachine_eject.plist:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC -//Apple Computer//DTD PLIST 1.0//EN http://www.apple.com/DTDs/PropertyList-1.0.dtd>
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>timemachine_eject</string>
    <key>Program</key>
    <string>/Users/username/bin/timemachine</string> <!-- Replace "username" with your username. "~/bin/timemachine" doesn't work -->
    <key>StartInterval</key>
    <integer>120</integer> <!-- run every two minutes for testing. -->
        <!-- Change this to a higher number like 43200 (run every 12 hours) once you've confirmed it works. -->
</dict>
</plist>

Make the script executable, unload the default plist, and load the new one:

chmod +x ~/bin/timemachine
sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.backupd-auto.plist
launchctl load ~/Library/LaunchAgents/timemachine_eject.plist

Any time you want to make changes to the plist file, you have to unload and load it:

launchctl unload ~/Library/LaunchAgents/timemachine_eject.plist
launchctl load ~/Library/LaunchAgents/timemachine_eject.plist

There might be a better way, but one solution might be to Applescript it. I haven't yet found any way to run an applescript after a backup, but you could:

1. Turn off automatic TM backup
2. Set up an applescript to run TM
   • Some googling turned up this line to force an immediate TM build: do shell script "/System/Library/CoreServices/backupd.bundle/Contents/Resources/backupd-helper >/dev/null 2>&1 &"
3. Add a line to eject the disk afterwards.
   • eject disk somedrivename

If you want this to happen on a regular basis, you could attach it to a cron job.