The links specified in earlier answers are dead or relocated, so here's the direct solution without having to run as Administrator:
- Exit MakeMKV if it's running
- Open the Windows Start menu
- Type gpedit.msc and press Enter
- Expand Computer Configuration > Administrative Templates > System > Removable Storage Access
- Double click All Removable Storage: Allow direct access in remote sessions in the right section
- Click the Enabled setting and OK.
- Start MakeMKV after a short while, e.g. 30 seconds.