Mkv format files. What format should I change them to before burning?

[davexnet]

Imgburn opens/closes the tray when it has finished writing in preperation for the verify step.
Works OK on full size internal drives that have a load/unload motor.

You may have to do it manually - like the message says

[jagabo]

Desktop CD/DVD/BD drives have an electric motor that opens and closes the drawer. So software can send a signal to the drive to open and close the drawer. Laptop drives are designed to be light so they don't have such a motor. They have a spring (and a small electronic latch) to open the drawer but no way to close it. So you have to close the drawer manually.

[cholla]

zipsick is using a PIONEER:BD-RW BDR-XS07U external USB.
It is a slim drive so the same as a laptop.
As jagabo posted they will open from a software like ImgBurn but have no way to close.