Crucial SSD 2.5" won't mount in any enclosure

What is the make & model of the enclosures you are using? I find the quality of most electronics these days to be quite poor so there are only a few brands that I will use personally. Personally I like these brands: Plugable, StarTech, and Anker. OWC also generally makes decent products, but not all of them allow access to a drive's health information so they are not always my favorite.

Are you connecting the SSD directly to the Mac?

If you are connecting the SSD directly to the Mac, then try using a good powered USB3 hub. Besides providing extra power to the drive the hub will also act as a filter & buffer. My Mac Mini actually requires this for reliable use of USB3 devices.

Have you tried booting the SSD externally on the 2012 laptop?

Have you tried both USB ports on the 2013 laptop?

FYI, Windows File Explorer won't see the Mac formatted SSD. You would need to use Disk Management on Windows in order to see if Windows physically detects the Mac formatted drive.

Also, check the firmware of the MX300 SSD. If you are using older firmware, then you should update the firmware to the latest version (easily accessible from the Crucial support website). While I don't believe this is the cause of your problems, I do know that the older firmware does have lots of issues so I highly recommend the firmware of the MX300 SSD to be using the most recent version available. The issues I've encountered using the older firmware is that when any of the SSD's NAND blocks go bad that they get stuck being reallocated so the SSD starts producing errors and it can lead to corruption of your data. Once the SSD gets into this state, it requires a firmware update AND an ATA Secure Erase to reset the SSD to factory defaults which destroys all data on the SSD. With the updated firmware I haven't had any problems with the MX300 SSD in our organization's Macs.

While backups have always been important, backups are even more important when using SSDs since it is impossible to recover accidentally deleted data from an SSD plus an SSD can fail at any time without any warning signs.

I've never heard of those brands before. The market is flooded with low quality products and Chinese knock offs. I would not surprised that they are the source of your problems. Try using an adapter, drive dock, or enclosure from one of the brands I suggested previously.

An SSD requires much better quality cables and electronics to support the faster speeds of the SSD which is why a hard drive may work fine, but an SSD won't work properly.

You are welcome. I'm really glad it was the adapter and not the SSD itself.

You really do want to update the firmware on that SSD if you have never done so though so you can avoid the problems I experienced with this model. (It really is a big problem and best to avoid if possible). Feel free to post the DriveDx report for the MX300 SSD so I can examine it for signs of the problem. Running the SSD's "short" selftest diagnostic is also a good idea. If your SSD shows signs of the problem I can guide you through "fixing" it.