"all estimate failed" on some partitions but not others

[David Newman]

An amanda server on Box A backs up partitions from amanda clients on Box A 
and Box B. The dumptype is gnutar for both boxes. Backups on Box B work 
fine. Backups on Box A fail with an "all estimate failed" error.

Both boxes run GNU tar 1.13.25. The disklist calls both boxes' partitions 
the same way. And amanda did back up Box A's partition when I used dump 
as the dumptype. (I'm only trying tar to see if there's a speed increase.)

Where should I be looking to determine why this is failing?
thanks
dn