Time Machine (external USB/SATA) on my 2-month old iMac had been working perfectly. I installed 10.5.3. Everything still works perfectly EXCEPT I get an occasional error with Time Machine's hourly backup saying it could not do the backup. I can do the backup manually and the subsequent automatic backup goes OK. It's happened twice since my installation of 10.5.3 last night. I did Repair of the external drive and no problems were found. Any ideas? Thanks.