www.makemkv.com

I run MakeMKV in multiple instances running simultaneously on the same machine (windows 10)
When this is in demo mode it works great, I open multiple instances, select each window, select a different drive in each, insert the discs, and each picks up the disc in the selected drive and works fine.
After registering with the beta key, every time a disc is inserted all instances reset the drive selector to the drive with the disc inserted most recently. Then each instance is trying to scan the disc at the same time leading to an eventual lockup since the drive can't handle that much seeking.
What about registering breaks this functionality?
For some reason each instance I open used to give the demo mode warning separately, but when using the key to register, I only do it in one instance and it works in all.

I run multiple instances with a purchased (permanent) key all the time.

The beta key isn't a permanent key, and it's the same key found in the installer for the current version (unless it's been more than a month between versions).

But the type of key used does not affect this functionality. Are you sure you closed all instances of MakeMKV, then started them for each drive?

Is MakeMKV asking you which drive to use as you start it? If not, close all but one instance, and turn on "Ask for single drive mode" in Preferences->IO. Close that last instance. Restart MakeMKV.