I wanted to make a backup of one of my external usb hard drives to another one (both are powered from the usb plug). they work nicely one-by-one, however, when both are connected to my notebook (Toshiba Satellite A60-185), after some short time one of them fails with I/O error in /var/log/messages, and then disappears completely from the system. This happened both on Kubuntu 7.10 and Kubuntu 9.04 (in the later, kde froze as well).
I thought that maybe my notebook can not supply power for the two drives simultaneously. However, from Windows I could copy several tens of GB without any problem. A fried of mine tried on her notebook, Kubuntu 8.04, there was no problem.
Does anybody have any idea, why? I want to make backups using rsync, so copying from windows is not a solution for me.