No, I did not tell that.
Unfortunately sometimes it just "disconnects", the device (phone, camera, whatever) loses the card without any tangible reason. Probably because the device was occasionally hit against something. Who knows?

[QUOTE-SW64](I was not able to figure out why this was so).[/QUOTE]

FAT was continually modified and maximum disk sizes were a thing. For a long time it was 512 MB. The result is the boot record changed. Those changes were backward compatible, but never forward compatible. That's the probable cause. Maybe format it in Dosbox?

Easy to understand.
The USB card reader has circuitry with code that makes it capable of reading the card. Some can only read cards of specific sizes or formats because of that onboard code. Most newer ones are designed to read the raw card but that has not always been the case.

Based on the previous replies, I now realized what I wrote were incomplete and inaccurate. Let me rephrase it for the sake of accuracy, in case someone else is troubleshooting a similar problem.

"I couldn't use just any USB card readers on this particular machine. It has to be a certain type of USB card reader before I could get the machine to recognize the card reader as a valid device. Once I've found a card reader that worked (and once I've resolved the incompatible partitions/formats on the cards that followed afterward), I was able to then read off and write to a memory card from this machine. Why that card reader and not another, I was not able to figure that one out at the time."

For what it's worth, that machine was Sony PlayStation III. It was very picky with these stuff.