Stop mac automatically trying to boot from external (non bootable) ssd device

Try a PRAM Reset (hold the PRAM reset for at least three chimes or a full minute if it doesn't have a startup chime), then make sure to manually select the default Startup Disk in System Preferences.

If you still have issues, then make sure to plug the SSD directly into your Mac bypassing any hubs, docks, or adapters/dongles. If you are already connecting the SSD directly to the Mac, then try connecting the SSD using a good brand of powered USB3 hub (with UASP protocol). A powered hub can provide more power to the drive and also act as a filter and buffer (I have to use a hub with my Mac Mini or my USB3 drives temporarily disable the USB ports on my Mini).

Make sure to test by disconnecting all other external devices except for your new SSD in case there is a compatibility issue with another device.

Try using another USB cable for the SSD.

Does the external SSD have a GUID partition? I would suggest using Disk Utility to erase the whole physical external SSD as GUID partition and exFAT. Make sure to temporarily move your data to another drive before erasing the SSD. If you still have problems, then use Disk Utility to erase the whole physical drive as GUID partition and MacOS Extended (Journaled) to see how it behaves. Recent versions of Disk Utility hide the physical drives by default so you need to click on "View" within Disk Utility and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility.

If none of this helps, then you may need to contact Crucial tech support for assistance. Also check the Crucial website to see if there is a firmware update for your external SSD.

Please let us know how you make out with it.

Good luck.

I'm suggesting there is a partitioning or file system issue on the external non-bootable SSD that is causing the problem, but @Barney-15E may also be correct that the internal drive has a partitioning or file system issue. Or maybe the internal drive is beginning to wear out or fail. Or you may be discovering your Mac has a problem that wasn't noticeable until you began to use the new external SSD.

We are suggesting things you can try to attempt to resolve the situation so you do not need to contact Crucial or return the drive. Troubleshooting is not exact science and it is not always quick and easy. Each thing we ask you to try may fix the problem or it may provide more information about what is happening. I've worked on servicing computers for over 25 years so I am trying to lend my own personal experience of troubleshooting odd & weird issues with you.

While what I am suggesting may not make sense as I don't always have time to completely explain my reasons for the suggestions, just know that macOS sometimes has problems with handling media formatted on other systems. The best thing to do is to use Disk Utility to partition & format the drive so you know for sure that the partitioning and file system is compatible with your Mac. By doing this you eliminate one possible cause to your problem.

It is very unlikely your Mac is attempting to actually boot from the external SSD, although the Mac will look to every connected drive to see if there is a bootloader on any of the external media (it will even scan the local network). More likely the Mac is having some sort of issue when attempting to read the external SSD which can be due to any number of reasons I've already suggested in my other post.

Have you set the volume you wish to boot from in Startup Disk.

An NVRAM reset will clear the current setting and set the internal as primary.

The fact that an NVRAM reset chose the BOOTCAMP volume as Startup Disk sounds like something is wrong with the partitioning of the internal drive. That may be why you are seeing what you see. But, I don't know for sure.

My guess as to why it tries to boot from the SSD is it tries to boot from the internal macOS volume and that fails for some reason, so it goes somewhere else.

I have personally found macOS to be very limited in some respects. I have had a lot of odd issues with macOS not being able to properly work with some drive configurations even when a drive has been erased using another Mac (yes Macs are that quirky). This does not surprise me at all. Keep in mind most third party manufacturer's are catering to Windows users. Users of alternate operating systems such as macOS and Linux are always an after thought and are usually on their own.

Personally I always re-partition and format all my drives. I will never trust a factory partitioned & formatted drive. You just don't know what might be on the drive that will cause problems.

I'm glad the solution was so simple for you. Glad we could help.