I have windows 10 installed on an external SSD, it started to crash so I assumed it was the SSD flaking out, as Parallels reported the disk was unavailable and the drive did disconnect from the Mac. What I did however was install windows 8.1 on the same drive and left it running all night and it was fine so drive is okay. I tried to decrypt the virtual machine and it has been saying less than a minute to go for the last hour and is stuck, is there a Windows 10 fault (fresh install) or something else going on?
This looks definitely like an hardware problem, just the fact that the drives disconnects on its own is a giveaway, that is completely unrelated to Parallels or Windows. Windows 10 is failing likely because the data got corrupted, Windows 8 hasn't failed (yet) because the data hasn't got corrupted, YET. If you can copy the .pvm containing Windows 10 to another storage device and try it you can get the definitive answer, and if it fails to copy then you'll know right away it is the drive. But it's very likely this drive will soon completely stop working, if you have important data in it backup it up ASAP, your drive is currently a time bomb that you don't know when it's going to explode, and after it does data recovery will be either expensive or impossible. Don't let yourself be fooled by the Windows 8 installation working, it proves nothing on the overall health of the drive. Also, consider a priority copying important data, the less you use the drive for less critical things the less time you'll have to copy the important ones, so if this Windows installation is not critical, don't use it, you are wasting precious time that should be used to backup data. If anything else, consider yourself lucky you had a warning, sometimes drives completely fail without any kind of warning. EDIT: I added a bunch of other info that I think you should know about the state of your drive.
