Backup to disk storage offline

Provisioning iSCSI LUNs and attaching the LUN to the Backup Exec Server (thus changing the NAS into a SAN devcie) is much the recommended way to configure B2D across the network (if your NAS device provides iSCSI support). The main reasons for this being

1) Security configuration - an iSCSI setup does not give the inherent problems that trying to get the service account credentials to work sometimes gives CIFS setups, as iSCSI maintains it's own security and does not try to dig into domain (or even worse local Windows SAM) based security

2) iSCSI protocols in theory have better performance than CIFs for backup streams

3) iSCSI protocols are typically more resilient with regards to network connection reliability. (which as per Craig V comment may be your issue.)

4) If you want to you can run Deduplication Storage across iSCSI (as long as it is hardware provided iSCSI and not installing a software iSCSI solution into an operating system as latency issues with some software iSCSI solutions have been seen to cause corruption of the  Deduplication Storage so cannot be supported)