when i use the add steam library folder the folder where the hard drive is mounted is not present
Not enough information (almost none)
to even begin to trying to help.
the drive is mounted at /home/davidchris/ManjPop! the manjpop folder doesnt show up on the home folder in the “create or select new steam folder”
There is threads here new members are recommended to read through.
“How to provide good information” is one of them.
There are zero tangible facts about your machine or your drive here.
I’m not one who can help with Steam related issues - never had it, never used it.
If anyone can work with the sparse info here - go right ahead. 
Please read:
If you add a Steam Library folder in Steam, you have to make sure the drive is mounted BEFORE you start Steam or else, Steam can’t see the folder… in your UNMOUNTED drive.