The situation with networked Time Machine backups is even worse than this - Apple won't even support networked Mac to Mac Time Machine backups over a wireless connection.
I had cause to call Apple support because I couldn't get my son's machine to recognise the Time Machine disk I'd attached to my own iMac. I thought I had a problem because the Apple web page that describes Time Machine states: "You can designate just about any HFS+ formatted FireWire or USB drive connected to a Mac as a Time Machine backup drive. Time Machine can also back up to another Mac running Leopard with Personal File Sharing, Leopard Server, or Xsan storage devices." [1]
Before I could even describe the problem, I had a very hard time persuading the UK Apple Support people I spoke to that a single Time Machine disk could even be used to back up more than one Mac; apparently they had been advising people to use entirely separate partitions if they wanted to use a single external drive to back up two or more Macs, and to carry the disk between them. Grudgingly, when I pointed out that the web page describing Time Machine also explicitly states that a single backup folder is created for multiple Macs they admitted that it was possible to use one disk to back up more than one machine, and that the connection between the two machines could be a network, because the page also mentions Macs running Personal File Sharing, but that didn't solve my initial problem.
After hanging up on the original call, I managed to resolve my original problem, which turned out to be incorrect privileges on the shared Time Machine volume. By this time, I'd done what the original Apple tech person suggested, which was to back up my son's machine with the backup disk connected via USB, which worked as expected.
Once I had got my son's machine to recognise the Time Machine disk over the network, which is via a wireless-n connection to an Apple Airport Extreme Gigabit base station, I attempted to get my son's machine to back itself up over the wireless connection. To my amazement, instead of using the existing completed backup, it created a completely new sparse image file, and attempted to back up from scratch. Very, very slowly. So slowly, it would take several days to back up the comparatively modest 40Gb of data on my son's machine.
Incredulous, I called Apple a second time. After mentioning how I'd resolved my problem, but that the network backup was unacceptably slow, Apple then told me that they wouldn't support using Time Machine from one Mac to another over a network, where the connection between the two Macs is wireless. This is despite my using all Apple branded kit, Time Machine being touted as a network capable backup system and Time Capsule being expressly sold on its wireless backup capabilities.
Time Machine was, for me, the feature that I upgraded to Leopard for. I've now been hugely let down by it twice - first when the ability to use any NAS device as a backup disk was pulled before Leopard shipped, and now, two system updates later, finding out that networked Time Machine backup still won't work unless there's a wire physically connecting the Macs together, I am very, very disenchanted, and my long-term loyalty to Apple has been sorely tested.
Steve Hubbard
[1]
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http://www.apple.com/macosx/features/timemachine.html>