Second drive gone bad within a few weeks, but it is a 2.5" HDD this time.  I 
run a couple of smartctl tests and sector 33428384 was reported to have a read 
failure; e.g.:
...
# 5  Extended offline    Completed: read failure       90%      1351         
33428384
# 6  Extended offline    Completed: read failure       90%      1350         
33428384
# 7  Extended offline    Completed: read failure       90%      1350         
33428384
# 8  Short offline       Completed without error       00%      1350         -
# 9  Conveyance offline  Completed without error       00%      1350         -

Then I took the drive out of the PC and run some tests again in a USB docking 
station.  First, 'hdparm --read-sector 33428384' returned success.  Then a 
short offline test returned "Completed without error", to be followed by a 
long test with the same result.  Interestingly, smartctl now shows:

"3 of 3 failed self-tests are outdated by newer successful extended offline 
self-test # 1"

However, the smarctl Thresholds table is warning "FAILING_NOW":

184 End-to-End_Error        0x0032   099   099   099    Old_age   Always   
FAILING_NOW 1

I'm running a reading test on it now to see if it reports any errors.  Given 
the results so far, is it worth keeping it around?  Perhaps for duplicate non-
mission critical data?