With databases, located on a raw device, nbackup tries to create difference file in /dev. For example (when difference file is not specified explicitly) when doing
nbackup -L /dev/sdb9
file /dev/sdb9.delta is created.
Or not created - if DEVFS is used. But even in my case (TMPFS is mounted on /dev) it may be quickly overflown by difference data. In traditional case (/dev is just part of root) - out of space is also quite possible, cause root is as for the rule not too big.
Solution is to force presence of explicitly set difference file location to start nbackup on raw device.