Why is my microSD card no longer readable on Mac mini?

rLo_101 wrote:

Is it possible the data loss on the external drive was caused by the tests I was doing with the Micro USB earlier? The external drive was unplugged during this time - it's reliable but quite old and not in its original housing.

Potentially, if you had accidentally selected the external drive in the Disk Utility and formatted it instead of the the micro SD card.

Even though the external drive was unplugged during your attempts with the microSD card, Disk Utility sometimes caches volume name assignments and mount points in a way that can carry over to newly attached devices — especially if the devices use similar partition schemes or GUIDs.

If, for example, you tried to erase or restore an image onto the microSD card but it failed halfway or didn’t actually complete a format, and then later you plugged in the rugged drive, Disk Utility may have misidentified the drive due to cached info or volume similarities and performed an operation without you intending to — especially if you accidentally selected the wrong volume or if the name “stuck” from a previous task.

The key now is how critical is that data on your external drive? If it is, stop writing to it, and then, consider using an app, like Disk Drill to try recovering the data.