We've previously run into hardware limitations (it was old hardware which has since been replaced). First, going from the big machine to Network Attached Storage had a couple of problems. The switches were setup incorrectly (half duplex instead of full, auto-negotiate was on, etc). Once they fixed those, we were running at 60-80 MBytes/Second. Not lightning quick, but acceptable for a one-off.

But it would only do that for 12 minutes and then taper off to just 2 or 3 MBytes/second. The problem turned out to be the bloody drives being used. It would handle the 60-80MB/Second until cache on the drive filled and the the darned thing would "go synchronous", which was a mode as frightful as old Apple Talk I.

Have your hardware guys check the path between your database and your disk target.