I remember these! And they weren't exactly circle CD-ROMs either, they had round sides but then straight top and bottom, so you could fit them in a business card holder / wallet. I always thought these were pretty cool.
Any drive that wasn't one where you insert the CD could read those, including portable music players of the time. You could buy mini CD'R's and burn all kinds of mini distros to it.
They came in all sorts of shapes. I distinctly recall jars of peanut butter having shirt-shaped ones in the lid for a while. Something like this[0]. As I understand it since CDs are written from the inside-out, you can make them roughly any shape you like as long as it's more or less balanced. You'll just lose capacity.
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