You might use smartmontools to check to see that the drive(s) aren't failing.
Figured it was, wikipedia said it was IA32, so maybe we need to make it i686 so there's no question.
How are you starting xfce4?What's your .xinitrc look like?
Chrooting is probably what you're looking for.
If you have a broadfailure card, you will probably need broadcom-wl-pf-core2.
It's probably the rpc-gssd, check dmesg.
Does your user have access to the optical group?
Have you restarted nfsd?
In a normal case, you probably don't need /mnt and /mnt/data unless they are in fact two separate entities (/etc/exports).
This is a nfs4 unmount bug.