When monitoring S.M.A.R.T. values of disks in a Unix system, smartmontools is usually the way to go.

Unfortunately monitoring external disk enclosures may be difficult or not possible at all. I haven't seen a firewire enclosure that supported SMART commands yet.

USB enclosures tend to work, but I noticed that the scheduled self-tests would not complete. For example, for the following disk a short (S) self-test is scheduled every day at 3am and a long (L) self-test every saturday at 6am:

$ cat /etc/smartd.conf
/dev/disk/by-id/scsi-SSAMSUNG_HD103UJ -d sat -a -o on -S on \
                           -s (S/../.././03|L/../../6/06) -I 190 -I 194 -W 5

But so far every test did not complete:

$ smartctl -d sat -l selftest /dev/sdb
=== START OF READ SMART DATA SECTION ===
SMART Self-test log structure revision number 0
Warning: ATA Specification requires self-test log structure revision number = 1
Num  Test_Description    Status           Remaining  LifeTime(hours)
# 1  Extended offline    Aborted by host      00%     25980
# 2  Short offline       Aborted by host      00%     25973
[...]

Someone else had a similar problem and suggested to run "smartctl -a /dev/disk... every few seconds while the self-tests are in progress, so that the disk would not shut down. Preliminary tests showed that this helped in my case as well.

From now on the self-test schedule in smartd.conf will be accompanied by some cronjob doing just this:

while smartctl -d sat -l selftest /dev/sdb 2>&1 | \
               grep -q "Self-test routine in progress"; do 
      sleep 30
done

The crontab(5) entry for the schedule above:

# m h  dom mon  dow   command
0   3    \*   \*    \*   script /dev/disk/by-id/scsi-SSAMSUNG_HD103UJ
0   6    \*   \*    6   script /dev/disk/by-id/scsi-SSAMSUNG_HD103UJ

We might add some fuzzyness to this "script" of course, so that it will work when the actual self-tests starts a bit late.