Can Emulators and Roms be run from an external hard drive?

Or do they need to be installed on my PC?
The Worlds Largest Rom & Emulator Website
https://www.theoldcomputer.com/forum/
https://www.theoldcomputer.com/forum/viewtopic.php?f=10&t=1495
gamfrkI wrote:for most emu's ive used (zsnes,epsxe,pj64 most used) all i had to do is make sure that they knew where the roms were