As you are trying to cerate a disk storage target for Backup jobs - then we recomemdn that if your NAS is capable of it you configure it to provide an iSCSI LUN to the Backup Exec server and then use that. The reasons for this recommendation are that it is much easier to configure the connection to the devcie itself AND in theory iSCSI should be a more optimized protocol in terms of performance for storage writes, than CIFs
If you cannot use iSCSI then it gets trickey as a number of things can deny the access:
1) The service account used by Backup Exec is the key credential for access to the CIFs share - so you must be able to login to the operating system console of the BE server itself and connect to the share using this login.
2) Once connected to the share as the service account, a B2Dtest must pass against the device
3) Because of point 1 - the NAS will usually have to be added so that it controls security access via Active Directory instead of maintaining its own list of logins.
4) The NTLM challenge response (security) levels on the Operating System of the Backup Exec server may need reducing if the NAS itself cannot support the same level as the OS defaults to.
Note: even understanding the above we have seen environments where customers have just been unable to configure shares on a NAS as a disk target for backups.