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I have always wondered how those worked. Do they copy through the computer and apply all the built in OS checks for accuracy of the copy, or do they do it all in the docking station using what kind of error checking.

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I don't bother with external bays, I just use Trueimage to an auxiliary HDD in the system, or one outside sometimes.
 
However those will work. They do block copies, so just clone everything, no awareness of file system or anything higher level. Error checking is all handled by drives themselves. They have fairly amazing processors that handle all of it. So a block copy works just fine.

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Oh I mean I don't bother with an external duplicator bay, I have Trueimage handle the imaging since it does file aware backups, compression, diffs, etc. Then sometimes I copy the images from the backup to an external manually. So I've not used a bay for disk duplication.
 
At work we do disk duplication often, but use computers with Symantec Ghost or Acronis Trueimage since they are filesystem aware and are faster (don't clone empty space), and can do things like clone a drive from a smaller to larger drive and grow the FS to use the extra space.
 
However straight block duplication, like these do, should work fine. I've done that on old drives with unsupported filesystems like XFS drives and so on and the clones are reliable.

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mike_mccue
 I am interested in having an appliance that can make the redundant backup without tying up the computer.

Do you never sleep?

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xcopy is built in to Windows and is incredibly useful once the first pass has been completed. I run it weekly or so on my C drive (SSD) to another HDD and just ran it yesterday... was ~1000 files. I have had cases where it has run ~15,000 files, but that is the higher end. It runs in less than 30 seconds most times to verify that any new(er) files are updated to the backup.
 
I would not waste the time to copy a million+ files each time just to watch one disk copy to the other. The "reality" is that 900,000+ files already there, so copying again is rather moot.