Hello tonebloke,
It isn't generally recommend to revert to a previous OS installation. The goal will be to determine why your external hard drive isn't mounting now. First, try restarting your Mac, then connect your external drive again: Ways to start up your Mac
If it still won't connect or mount, you may need to be repaired. You can try this from Disk Utility:
How to repair a Mac disk with Disk Utility
If your external drive still won't mount in Disk Utility, next would be to test the connection to another port, using another cable, or even testing the connection with another computer. That will narrow down where the issue might be coming from. Reply back with how it goes.
Take care.