Just something you might want to look into since Sierra no longer gets security updates. I have thought of this. My iMac has 16GB of RAM and I boot from a Samsung X5. Now, my concerns about upgrading to latest OSX (macOS Big Sur??) is that this will change the format of the Samsung X5 from APFS to HFS+. However, the hard drive inside my iMac (a fusion drive) will still, of course, be formatted as APFS. If I understand things correctly, I will not then be able to boot my iMac from the Samsung X5 until I also upgrade my iMac itself to the HFS+ file system, which I do not think will be possible as it is a Fusion drive as Fusion drives remain as APFS. Actuallt, if I upgrade my iMac with its Fusion Drive to Big Sur, try Parallels and don't like it I guess I could Carbon Copy Clone the version of my system currently running on my Samsung X5 to my internal iMac Fusion drive. Correct?