FYI,

After a server crash during a backup to a Truecrypt encrypted ZFS pool on an external disk I could not import it again after the forced reboot because of the following error:

$ sudo zpool import backup1
cannot import 'backup1': a pool with that name is already created/imported,
and no additional pools with that name were found

$ sudo zpool status
pool: backup1
state: UNAVAIL
status: One or more devices could not be used because the label is missing
or invalid. There are insufficient replicas for the pool to continue
functioning.
action: Destroy and re-create the pool from
a backup source.
see: http://zfsonlinux.org/msg/ZFS-8000-5E
scan: none requested
config:

NAME STATE READ WRITE CKSUM
backup1 UNAVAIL 0 0 0 insufficient replicas
loop1 UNAVAIL 0 0 0 corrupted data

I did not have to destroy and re-create the pool from a backup source as suggested because after commands:

sudo zpool export backup1
sudo zpool import -f backup1

The pool was online again.

Regards,
Gijsbert