No fstab entries. Just delete the default rom folder, plug in a usb external hdd, and symlink the rom folder from the hard drive to where the old folder was as outlined here: https://www.petrockblock.com/forums/topic/usb-rom-storage-curiosity/#post-88737
There’s no need to permanently mount it, as long as it’s plugged in it’s mounted and the roms are pointed to the right place.
I have 45gb of roms at the moment with more expected to be added, and would rather not have to readd them or keep a backup elsewhere for if/when I wipe the sd card for future Retropie images. Also, no microsd card on the planet can hold all of the Playstation One images. A potential side effect is the external hdd is faster than the microsd, but I couldn’t tell you if it has made a difference.
When I was searching for how to keep my roms on the hdd, it seemed to be fairly commonplace.