Time Machine backup to Airport Extreme AirDisk I thought it was NOT possible to use the USB hard disk attached to the Airport Extreme Base Station (AEBS) as a backup destination for Time Machine backups. This was mostly due to Google results as well as this Apple article.

I did the following (no hacks, nothing, just straight up!)


Formatted a 1.5 TB hard drive as 1GB + 0.5GB, both HFS+ Journalled on a GUID partition scheme (did this on my Mac)
Attached it to my 5th gen AEBS. 
Back on the Mac, I navigated to the AirDisk volumes via Finder to mount it (i.e. Finder => AEBS => the HFS+ volumes)
Opened Time Machine (on Mountain Lion, 10.8.3) and hit select disk. I saw that Time Machine was asking me if I could use the USB disk attached to the AEBS for the backup destination!!


I went ahead and picked it and the backup is running just fine (images below).

EDIT: Well well, I didn't notice this before but TM seems to think it's backing to a Time Capsule! I don't recall doing anything special to either my Mac(s) or the AEBS. The AEBS was recently upgraded to the latest firmware (7.6.3 or 76300.7 it toggles when the version is clicked)



For a second confirmation, I went to my other mac (also Mountain Lion) and the same result - TM allowed me to choose that AirDisk for backup. Interestingly two separate sparse bundles were created on the on the AEBS Time Machine AirDisk volume, one for each Mac's TM backup instead of trying to cram both into a single sparse bundle. Not sure if that was because I enabled encryption (which I did) on the TM backups.


  Question: Is the [AEBS + USB/AirDisk + Time Machine] combo finally, officially supported? On Mountain Lion or something? I couldn't find documentation along those lines but
  since we're talking about backups, I just want to make sure I'm not
  following an unsupported mechanism.


Disk selection dialog:


And the actual backup itself progressing:
Question: Is the [AEBS + USB/AirDisk + Time Machine] combo finally,
  officially supported? On Mountain Lion or something? I couldn't find
  documentation along those lines but since we're talking about backups,
  I just want to make sure I'm not following an unsupported mechanism.


It was not supported as of Mountain Lion (see OS X Mountain Lion: Disks you can use with Time Machine) but according to the newest KB article (Use Time Machine to back up or restore your Mac) it is now officially supported:


  Time Machine is the built-in backup feature of your Mac. To use it, you
  need an external storage solution, sold separately:
  
  
  External hard drive connected to a USB, FireWire, or Thunderbolt port on your Mac 
  Time Capsule or macOS Server on your network 
  External hard drive connected to the USB port of an AirPort Extreme base station on your network  
  


I couldn't find any information about whether this functionality was added as an Airport Express firmware update or was introduced with a specific OS version (or both), and thus I can't provide a firmware/OS version support matrix, but from 
Backup disks you can use with Time Machine the minimum requirements seem to be:


AirPort Extreme 802.11ac model
Yosemite (OS X 10.10) (the earliest OS that references the article above, see OS X Yosemite: Back up your files with Time Machine)