www.makemkv.com

I searched the forum, but didn't find anything exactly the same.
I was trying to rip Stargate (1994), and unbeknownst to me, autoplay was configured to launch PowerDVD17 when a disc is inserted.
The rip operation suddenly began claiming the USB blu-ray drive was not present (an LG BE14NU40). Since powerDVD17 was launched, I knew autoplay was enabled on this drive.
I set autoplay to no action, and my troubles disappeared.

Why doesn't autoplay realize makeMKV already has the device open?

That's a question for Microsoft, really. Of course, Macs and Linux machines often do it, too, when you have autoplay enabled. MakeMKV does not open the device for exclusive access, as far as I know.

How do you turn autoplay off?

On Windows, open My Computer or This PC (whatever shows you icons for every drive), then select the drive, and right-click to get pop-up menu, select menu item Open Autoplay, and select Take No Action.