Welcome to the community, @pjbadger!
I'd suggest you check how the HDD is recognized in
Disk Management in Windows. Sharing a screenshot would be really helpful too. You mentioned that it wasn't booting when you removed the drive, so I'd suggest you also
run its brand-specific diagnostic tool or a third-party alternative to determine the health and SMART status.
Some enclosures would require you to format the HDD due to the differently configured LBAs in them, so you should also check how the hard drive is recognized if it's connected internally via SATA as a secondary drive in a PC.
Make sure you backup all your important data from the drive as soon as you gain access to it!
Keep me posted with the troubleshooting!
SuperSoph_WD