Playing GMod on an external hard drive?

Hey guys, recently I bought GMod off Steam and I wanted to download it to my 1 terabyte hard drive as I was running out of space on my laptop and I figured every time I wanted to play I’d just plug in my hard drive and start up Steam and GMod off of that. Steam gave me the option to install GMod to my hard drive so I thought it would allow me to play off it. So the first two times GMod started up fine and I downloaded some addons and was going just fine, but today I have plugged in my hard drive and started up GMod and Steam is telling me I need to reinstall GMod even though just yesterday it was working fine. So I’m just wondering if anyone else has had this problem and if I am able to play my GMod straight off of my hard drive? Also I use a Mac btw.

Any help would be GREATLY appreciated thanks.

Make sure you mount the harddrive in the Steam preferences.

Thankyou so much man I tried it and it worked just fine really appreciate your help