Peter Sichel (of Sustainable Softworks) in a comment at MacInTouch:
There is an option to have Time Machine show other network attached disks:
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
But Apple is hesitant to enable this as the default since it places the integrity of interrupted Time Machine backups at risk. So Apple is in a difficult spot with respect to supporting 3rd party NAS devices.
I heard a similar explanation from a few people at Macworld Expo last week, albeit regarding AFP, not USB — more or less that the AFP implementation in current AirPort base stations isn’t robust enough. Backups seem to work, but fail when you attempt to restore from them.
★ Thursday, 24 January 2008