NetWare backup to disk fails

This is kind of a weird issue. I already opened a service call with EMC support and since that the client is already de-supported, so didn't get much help. So I'm now redirecting my issue to the expert community forum of Networker Admins and users,

NetWare file server backup client with 1 single volume of 2TB with millions of small file, FULL and INC backup to disk fails, but when re-directed to tapes, it works or the backups completes. This is the error messages when the backup to disk fails:

1 retry attempted

SAVE.NLM: (TSAFS.NLM 6.53.3 266) This data set is in use and cannot be accessed at this time. :

SAVE.NLM: RPC error: RPC send operation failed.  A network connection could not be established with the host.

SAVE.NLM: RPC error: Cancelled

Backup environment:

Backup Server: Red Hat Linux v5.8

Networker v6.3.3

Disk Device: Quantum DXi7500, shares NFS mounted

NetWare client O/S: v6.5 sp8

NetWare networker: v7.2.1

thanks in advance for any help/suggestion.