Fbackup to multiple files

I suffered a fault on a tape drive and would like to redirect my fbackup from /dev/rmt/0m to a NFS Share. I have mounted a NFS share in /mnt/NFSShare. When I run the following command:

/etc/fbackup -0 -uv -i /backup2/BEM/BEML1 -c /usr/local/scripts/backup/fbackup_config -f /mnt/NFSShare/BEML1.fbackup

With this config file:

blocksperrecord    128
records            32
checkpointfreq     256
readerprocesses    2
maxretries         0
retrylimit         5000000
maxvoluses         10000

It gets to about 21GB in about 15 min then slows down o about 700MB / hour.
Is this to do with the size and if so can I specify multiple files with the "-f" option and how can I limit them to 20GB each?
Thanks for any help.

Dont you have another remote tape drive?

I having to "backup" on NFS I would go with other tools than fbackup... or insure you backup all you want within a hour max if not split the taks into multiple fbackups not exceeding 1/2 hour or it risk to last for ages... ( thats mostly because priority drops with time passing... ) the best performance being withing 1/4 hour...
Ever though of using pax?

I don't have a another tape drive. It's the only HP-UX box I have.
Time is not a problem as long as the job completes in 5h.
I'll first try splitting the job as I am waiting for a replacement drive and it's the recommended software for backing up our system by our ERP software vendor.

Thanks