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](http://support.apple.com/kb/HT2038?viewlocale=en_US&locale=en_US).

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

1. Formatted a 1.5 TB hard drive as 1GB + 0.5GB, both HFS+ Journalled on a GUID partition scheme (did this on my Mac)
2. Attached it to my 5th gen AEBS.
3. Back on the Mac, I navigated to the AirDisk volumes via Finder to mount it (i.e. Finder => AEBS => the HFS+ volumes)
4. 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)

![enter image description here](https://i.stack.imgur.com/SwqYp.png)

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:
![enter image description here](https://i.stack.imgur.com/CUDz7.png)

And the actual backup itself progressing:
![enter image description here](https://i.stack.imgur.com/xHdpd.png)