I have an iMac 2.16 GHz Intel Core 2 Duo running Mac OS X 10.4.11 (Tiger) and a MacBook 2.1 GHz Intel Core 2 Duo running Mac OS X 10.5.5 (Leopard). I recently purchased a Seagate FreeAgent Desk for Mac 1.5 TB external hard drive to store digital video files and to serve as a Time Machine backup drive for the MacBook.

Following advice on this forum and elsewhere, I first partitioned the drive so that TM wouldn't eat up the whole thing .. I gave the TM partition 300 GB. Then I connected it directly to the MacBook via FireWire 400 for the first TM backup, and excluded all but the Users folder. The first backup went without a hitch, backing up 11 GB (there's only 1 user account).

Then I connected the FreeAgent to the iMac via FireWire 400 (remember the iMac is running Tiger), and from the MacBook connected to the iMac as an administrator to get the TM volume to show up under Devices in the left pane of the Finder. Once it was there, TM found it and allowed me to browse past files. But it has not added another backup since then; each subsequent backup fails with the message that it "could not mount the backup volume." The TM volume shows only in Finder; it does not appear on the desktop or in the TM "choose disk" list.

Here's my question: Should this work, or do I also need Leopard on the iMac to which the external drive is connected? If I don't need Leopard, then what did I miss?

Thanks for your help.