So here's a recent Tecmar drive sale on eBay.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250993088572

So the seller says this:

Tecmar made several versions of the Mac Drive.  All of them shipped with at least one 5 megabyte removable cartridge (Winchester-type) drive in the lower slot.  The upper slot could be empty, filled with a second 5MB cartridge drive, or filled with a 5 or 10 megabyte hard drive.  This particular Mac Drive has the 5MB cartridge drive in the lower slot and an empty upper slot.  I do not have any Tecmar cartridges, so I am unable to test whether the cartridge drive works beyond seeing that it is found by the software.

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This suggests that a compatible drive could be popped in and formatted by the Tecmar software. Anybody got any idea what kind of drive would work in this? I'm assuming MFM, or ST-506 compatible? I'm sure the manual indicates. Unlike Apple Tecmar probably has a section on user servicing.

What would be the purpose of having a tape drive without a hard disk to back up? Or was it designed to back up your floppies as well as a hard drive? I wouldn't think it could be used in real time. My guess is that the hard drive was removed at some point for use in a faster SCSI device.

FYI Bunsen, I'm getting a disk image of the software for us to dissect.