iTunes problem with updated ipod

Greetings!
i've update my ipod(4g) to ios 5.1 with restoring with another itunes and now i wanna get back my data from my backups on my own macbook but everytime i connect my ipod to mac, itunes will detect it as a new device. it gave me couple options: define as new device or restore.
when i choose to define it as a new device it will give me 54 error while im trying to sync it.
what's the problem and what should i do?
Thanks

What about backing up to iCloud straight from your iPod, then syncing with iTunes, the restoring back from iCloud?

thats not gonna working too. its not about how i've backup, its about the 54 error and detecting ipod in itunes as known device