Lot of confusion here: in userspace there is only one filesystem starting at / (no notion of drive C: drive D: on linux), all users and programs (with the right priviledges, for security reasons) can access the entire filesystem. Moreover with LVM there's a lot of flexibility in handling how the filesystem utilizes your storage. You can combine together disks and partitions in physical and logical volumes so that the size of a sub tree of the big filesystem starting at / (sub directory, for example /home) is not constrained by the size of a disk or partition. So for instance you can add the free space of your 120GB ssd to your logical volume hosting your /home (your home will grow by 120GB) and so for your big hard disks. To your other point: NEVER EVER log in as root , expecially if your system is connected to a network, user root is there for accomplishing administrative tasks. Log in as regular user and use sudo from the command line for that or enter a password when asked by some priviledged proccesses.