I am not sure of your setup but
if you have an empty drive bay in your system
you could use it to perform your backups to a single drive
(or one drive for each day of the week)

I have tried this to backup to an external USB (single) drive as well, but have never tried it with a external drive with RAID device like the buffalo mini nas.


My other question is are you trying to back NAS shares or iSCSI Target ?
for NAS this will work

if you are trying to backup iSCSI Target you will need to do something else.
check the forum, there are a lot of posts about it