how can I fix disk permissions on a headless Mac that won't boot?
Two options:


You can repair permissions using Target Disk Mode from your MacBook - as you mentioned, you'll need a FireWire or Thunderbolt cable to do so. No need to use the recovery partition on the Mini - you can use Disk Utility on the MacBook to repair the drive on the Mini. In order to use Target Mode on a headless machine that won't boot, hold the 'T' key down during startup.
Alternatively - since you need to buy a cable, you could instead buy an adapter to connect a monitor (if you have one) &amp; repair using the recovery partition on the Mini.


I'd have both options on hand in case of issues. Target Disk Mode will work if you have a keyboard &amp; a FireWire cable.