Connected External SSD Drive Suddenly Wouldn't Allow My Mac to Boot Up, Then More Strange Issues

I have a number of external drives (both SDD and HDD), and I use many of them as backups for my photographs (which are large: often 2GB-3GB each). Most of them are connected to my iMac 27" 2017 via a CalDigit dock that is plugged into my iMac to the Thunderbolt port.

A month ago, I bought a Crucial 4TB SSD 9 Pro to use as a backup as well (I have an original set of images on my internal drive and I'm starting to get the beach ball often as my internal drive fills up). Everything seemed to be going well with the whole system until this past weekend. First thing I noticed was that my Time Machine external HDD kept ejecting, coming back, ejecting again, etc. Did some sleuthing and discovered the power plug had somehow started to come out of the plug, but just about halfway (which would suggest the reason my drive kept ejecting and reconnecting). I plugged it back in securely.

I then decided to reboot my computer. But when I did, I just got the Apple logo, and it went no further (no progress bar). I unplugged all my drives, and it booted up fine. So, one of my externals seemed to be the culprit. I isolated it to the Crucial SSD. If I booted up the computer with it disconnected and then plugged the SSD in, it mounted.

Next step: I ran First Aid on all drives (including my internal 2TB HDD). All were fine except the Crucial. Here's what I got:

Now, whenever I mounted this SSD drive, I got a warning saying that the drive needed to be erased and to move my files off the drive before erasing. I tried trashing the file that apparently had too few clusters for its size (what the **** does that mean?), but I could not trash any files on that SSD disk (I could move them to another drive, though). As I mentioned, I have a number of identical backups on different drives with this file on it, and all drives were verified.

So, what was it about this drive that was causing the problem? Was it even this file that caused the problem? Did the loose plug on my Time Machine drive start the problem—the drive self-ejecting and remounting? Or was the drive simply defective?

I've returned the drive to the place I bought it and they are sending me a replacement. But I'm trying to get a handle on what caused this drive to fail so I don't repeat the problem if it was user error. I called Crucial and spoke with tech support. They couldn't give me any definite answers other than to say the drive was probably defective. They did say that it was "plug and play" for my Mac. I know it was formatted as ExFAT, but not whether it was APFS. When I get the replacement drive, I will reformat it before I store anything on it.

I also did a little sleuthing about clusters. Basically, they're about the size allocation of the sectors (I believe). I started wondering if my files were too big. But the tech support person said no. And I've had no problem storing them on my Seagate SSD drives.

Can anyone illuminate me about the note saying that my file had too few clusters for its size? What does that mean? Also, can anyone tell me what those error codes mean? Calling Apple Support, they didn't know.

And can anyone suggest anything I should be doing before I start storing files on this drive? I'm only using a Mac, no Windows in my work.

Thanks.