Kind of technical for me but I did use the OS on the external drive a few times. After this I think I had the Mac set to boot up to the internal drive. I always leave the Mac on so rarely did I have to reboot.Is it possible that you have multiple partitions (or in APFS speak: volumes, but it seems that your external SSD is HFS+-formatted) on that external drive? You say you never really used the OS installed on this drive but rather utilized it for storing data. It's feasible that you have a separate partition that you will have to repair and then mount manually since it might have become corrupted.
The Samsung does show 286gb used and 213gb Free. And right now I know for sure I can’t “see” anywhere near 286 gb.
I also found this re. The ST_M13FQBL a media. “is usually an indication of a fault caused by bad sectors on the drive”
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