Let's assume a PC has 2 optical drives, of which one can write DVDs and the other one is a DVD-ROM drive. I have a live Linux Mint Mate 18.3 Sylvia x86_64 on a DVD-RW. (If anyone is interested: It is a Maxell RiTEKW06 with ×6 recording speed).
I booted Linux Mint with the burner drive because it can read DVD-RW's faster (×12 CAV to be specific) than the DVD-ROM drive (×4 CAV only).
Now I need to burn a DVD but I do not wish to restart the PC, just in order to make Linux use the upper drive for SquashFS (live session file system).
I used the example to make it understandable what I mean in a possible real-life situation. In reality, I tested whether Linux can be booted from an external DVD drive, connected via USB.
Now I need that port, so I wish to swap the live DVD to the internal laptop DVD drive.