How can I make an external drive behave as an internal drive?
I just found this answer, sorry I tried looking first but couldn't find it....

You should be able to do this with dd by setting Removable to False.

Unmount your disk, then run…

dd if=/dev/zero of=/dev/disk2 bs=1 count=1

Don't forget to replace disk2 with the actual disk identifier. 

This command will erase the contents of the disk. Untested, be careful, usual 'not responsible if stuff goes wrong' applies :)