Jovey, are you using an NetBackup for MS SQL policy type etc?
I've seen even average spec servers as NBU clients sending 80+ MB/s via LAN to a media server, for databases in the 120GB size region.
Can you post your "batch" file?
How many databases are there?
Pls describe the LUN/volume that houses the 420GB database - what connectivity, RAID, number of spindles etc...