I recently noticed that Time Machine on both my Macs (Mountain Lion, 10.8.3) seem to think that my Airport Extreme base station (AEBS) + USB hard drive is actually a Time Capsule. Screenshots below. BTW, I asked this question on the official support status of AEBS + USB HDD, but this question is different (though related)
Question: Why does Time Machine (on Mountain Lion) think that an AEBS with an external HFS+ USB drive is a Time Capsule? I don't recall doing anything special but are there some special settings that "make it so"?
TM thinks it's backing to a Time Capsule
TM performing a backup to the USB HDD (HFS+) attached to the AEBS
Best Answer
Apple only supports backup to Time Capsule, so the wording there is a constant and not something designed to know it's not backing up to Time Capsule as opposed to an actual server or directly connected drive.
You can test this (since I doubt they recently changed course on this with an update) by connecting a totally new drive to the Airport Extreme, wipe the drive from AirPort Utility and then try to set up Time Machine to back up to that newly erased drive.