"Device can not be read from or written ...

I have recently updated to the 3.1 firmware on my iPhone 3G (after a couple failed attempts).
The problems seemed to start there when the update froze midway through and I had to try a couple more times. After I did and restored my settings, iTunes said that my backup file didn't work. I tried a couple more times to restore from the backup but ended up having to restore as a new device.
Now, as I try to sync all of my, well, everything, I keep getting an error message.
It synced all of my contacts, apps, calendar, etc., but when it gets to my music and videos it will sync a bit of it before giving me the error messages:
"Cannot sync _____, the required disk is not found"
and immediately after,
"Device can not be read from or written to."
I've tried syncing my iPhone many times, and these errors keep coming up.
What should I do?

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