I'd expect that. Reboot and then backup.
I have approximately 52Gb music that I back up on an external hard drive. I've backed up 2 or 3 times before with no real difficulty, but tonight I'm running into a troublesome error message.
The message reads: "cannot copy file: cannot read from the source file or disk"
It stops the copy process once the message pops up, and does not specify what file is causing the message.
I'm using Windows XP and backing up from Itunes version 8.02 onto a 120Mb external hard drive