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This will show all mounted file systems.
You should see several /dev/sdax entries - if not, you are just looking at the the USB.
To mount real disk entries you can do the following:-
sudo mkdir /mnt/mountpoint
sudo mount /dev/sdax /mnt/mountpoint
Where mountpoint is a name of your choosing and sdax=sda3 or sda5 or whatever is the corresponding partition.
In particular do the following for your / partition:-
sudo mkdir /mnt/system
sudo mount /dev/sda? /mnt/system
(where /dev/sda? is your partition with / in it).
Send the output of that last 'cat' to the forum.