Can't install on external HD (mac) -- error /cdrom won't unmount

System: Macbook Air (late 2015), 1T SSD external, 128gb SD card with Budgie flashed.

I partitioned the HD and when I run install I get the error that it can’t be installed because /cdrom will not unmount.

Is it safe to unmount /cdrom from inside Budgie flash? If so, how would I do that?