if it detects no filesystem, there should be an entry for "diskXsY" where x and y are numbersw. I used this once to image the firmware of a SanDisk MP3 player.

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Did you use Snow Leopard's Disk Utility to image the SanDisk firmware (and what filesystem was it)?

Here's the result of my experiments with the ZIP drive. Fresh from booting a Mac Plus, I inserted an MFS ZIP disk. Disk Utility seemed to really have a problem with the disk (lots of spinning beach balls). Finally I was able to select the grayed-out volume and tried to make a disk image. The progress bar appeared and then some time later I got an Input/Output error and the image failed.

So, this could be a problem the Snow Leopard Disk Utility has with ZIP disks in particular (after I ejected the disk, it would no longer boot the Mac Plus either), or it is a problem with Snow Leopard refusing to create a disk image for which it does not support its filesystem.

Someone with a USB floppy drive & Snow Leopard should test a few unsupported formats and see what happens.