Thanks for everyone's input.  I took your advice not to try cleaning the disks 
but as I was using ext2, befor discarding them I decided to try a journaling 
file system just in case the errors were due to errors when foolishly 
switching of the power or similar.
I tried ext3, but after unmounting the disk, fsck still stalled.  I then tried 
reiserfs and this time fsck was OK and ran to completion.  Read/write speeds 
through the usb connection were slow so I used the distructive form of 
badblocks of the first 100 Gb (~8 hours) and then tests of short patches every 
100 Gb.  These were all OK.  Reinstalled reiserfs and copied across about 30 
GB of real data - fsck still OK.  Tried another 60 Gb but this time fsck 
failed because it was unable to read one of the blocks.  There was a nice 
message:
------------------
The problem has occurred looks like a hardware problem. If you have
bad blocks, we advise you to get a new hard drive, because once you
get one bad block  that the disk  drive internals  cannot hide from
your sight,the chances of getting more are generally said to become
much higher  (precise statistics are unknown to us), and  this disk
drive is probably not expensive enough  for you to you to risk your
time and  data on it.  
------------------
I need new archive sytem but what is the best way to do this? One good 
suggestion was to use disks from different manufacturers, or at least 
different batches from the same one.  However as archiving only takes 10 
minutes or so every two weeks (I use rsync) system robustness when heavily 
used is not as important as lifetime. So, given that I have about 400Gb, and 
would like to have a 99.9% chance of still holding most of the data in ten 
years time,
1.  Are disks still my best archive storage system?
2.  Should I just be buy cheap disks and replace one of the pair each year? 
3.  Should I be spending as much effort checking the disks as archiving?
4.  Is there a better strategy?
Regards,
David Webb.
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