pass0 device seems to not work

[Justin C. Sherrill]

Burning a DVD seems to have stopped working on my 1.4.4 DragonFly system. 
This used to work, though since my backups to DVD are scattered between
home and my office, I can't pinpoint a time at which this stopped working
by looking at the last one.

# growisofs -dvd-compat -Z /dev/cd0c -J -joliet-long -R /home/backup/
Executing 'mkisofs -J -joliet-long -R /home/backup/ | builtin_dd
of=/dev/pass0 obs=32k seek=0'
:-( /dev/pass0: 2295104 blocks are free, 2910600 to be written!
:-( write failed: No space left on device

I tried doing this with a manually-created ISO:

# growisofs -dvd-compat -Z /dev/cd0c=test.iso
Executing 'builtin_dd if=test.iso of=/dev/pass0 obs=32k seek=0'
:-( /dev/pass0: 2295104 blocks are free, 2910600 to be written!

My first guess was that pass0 was writing to, say, /tmp, but growisofs
does not do that.  I'm unsure of how to troubleshoot farther.