After installing the update 10.5.2 for Leopard, Time Machine (working correctly since October 2007) started to fail. The problem seemed related to certain files which couldn’t be copied. I wasn’t able to solve the problem even if I excluded them from backup. Moreover, there was a problem connected to UUID (as reported in console log).
Last night I’ve found the solution in Apple Support Discussions. I have an iMac G5 with an external iSight and a WD 500 Gb external hard disk. There seems to be a conflict between these two units. Everything was fine before 10.5.2. I unplugged my iSight and everything went fine.
Apple !!!!! Conflicting peripherals are not acceptable in 2008. Anyway, I have to say that I had the same problem a year ago between iSight and another firewire external hard disk. The problem was related to my iMac series (there was an hardware problem connected to the power supply: see iMac 2005 – Power supply issue on Apple Support). Apple assistance replaced my power supply and everything was fine.