Mirrored RAID using 2 external USB drives

[jscordia]

Hi guys. I'm new to the forum, and hoping you can help me.

I have a single 1.8GHz G5 with OSX 10.4, all software up to date. I bought 2 300gig Seagate external USB drives, mounted them OK, formattted them Mac OS Extended and then using Disk Utility *tried* to set them up as a Mirrored RAID. They are supposed to join to become my main itunes/iphoto etc automatically backed up disk.

In Disk utility you're supposed to simply drag the disks you want to RAID from the LHS window (where they show up ok) to a window on the RHS, but no joy.

Is there something about external USB that the system doesnt like?

BTW, also bought a 3rd party USB 2.0 x 2 PCI card. Seems to work ok. However in case that was my problem, I currently have the drives plugged into the back of the Cinema Display. Again, is using a hub a problem? I'm short of USB ports!

Thanks in anticipation for your help.

Cheers!

Jon