External SSD has become unmounted. How to remount?

FWIW, that hub only has a 63% five star rating with only 76 reviews which does not bode well for its reliability. As a matter of fact, scanning over a dozen of different hub listings on Amazon do not show any with really stellar ratings.

Since the Minis are new and these hubs are new, perhaps it is just growing pains with these hubs or just endemic of their quality. I would be willing to bet that the vast majority of them are made by one company (at lest the internals) and simply rebranded as is quite often done. With that the case, you could have common issues across many brands.

With that said, I have found with any enclosure or hub with SSD capabilities that Samsung 980 and 990 Pro and Western Digital 850 Black to be the most reliable.

What enclosure do you have the Crucial M.2 SSD installed in..?

How did you initially format the Crucial M.2 SSD..?

What are you using the Crucial M.2 SSD for..?

see > Initialize an external storage device in Disk Utility on Mac - Apple Support

First try a better quality USB-C cable between the Mac mini and Hub.

(it does not have to be a 6 inches cable)

Mac OS Extended should work for your Photos Library.

However APFS would have been the best choice with the latest macOS versions.

If you decide to reformat the SSD APFS, be sure to copy your Photos Library to another drive first.

Initialize an external storage device in Disk Utility on Mac - Apple Support

If that doesn't help, then contact the Pulwtop Hub manufacture for support, a replacement or refund.

https://pulwtop.com/products/pulwtop-mac-mini-hub-bd240a

👍 Good points, woodmeister50.

I have a similar hub (RayCue). Your NVME drive should be formatted to APFS, GUID. I'm using a WD SN550 drive and no issues.