Ok I can help seeing as I have just figured it all out without smartalec comments from the peanut gallery.
If you are having problems with the PMS not seeing your secondary / external drives check this out it is not that hard to do, this is for linux mint
open the Linux 'MENU' Find 'PEFERENCES' , then DISKS
once you open disks choose the disk you are using for your media storage, once that is highlighted look for the settings ( 2 gears in mesh) , now open 'EDIT MOUNT OPTIONS' look for 'MOUNT POINT'
write down the mount point entry which should look something like this /mnt/75c3e8666n5f6f6 <= this would be how Linux recognizes the drive.
now you go back to PS3 Media server , Navigation/Share Settings , under 'SHARED FOLDERS' you will see a GREEN Plus Sign where you can add shared folders.
a box pops up and near the bottom you will see 'FOLDER NAME' in the entry box you will type that MOUNT POINT address you wrote down that looks like this /mnt/75c3e8666ff6f6f6 and you will add the folder name where your shares are /downloads or what ever the name is, it should look like this in that box /mnt/75c3e666fff6f6/Downloads click save on the top before you lose it.now you can do this with external media as well just get that mount point address and add your shared folders name and save.
I hope this helps you and saves you the frustration it has caused me . LOL