File systems and mount points
You're missing out on a very important way Ubuntu/Linux handles file systems: all comes down to the same root (
/) and other file systems ("volumes", "disks", "partitions") will be mounted on a location inside the root.
To explain two most interesting lines:
/ is full:
/dev/mapper/eubuntu10x32-root 3.5G 3.3G 0 100% /
Here you see a "device mapper" meta-device being used for your root filesystem. I think you're using LVM here or encryption.
3.5GiB for a
/ is a very low capacity for a desktop installation, unless you have directories like
/var moved out of it. In your case it's clearly not sufficient.
Your second disk (
sdb, without partitions) is mounted at
/home and a partition of
sda is mounted as
/dev/sdb 50G 27G 21G 57% /home
/dev/sda1 228M 35M 181M 17% /boot
/boot is common for a LVM/encrypted installation to allow the bootloader (Grub) to load the relevant software/drivers to be able to reach your other file systems to actually boot the operating system.
So, basically, your
/boot aren't just directories. They are directories on the
/ file system, but there's another file system mounted on top of it. This means that whenever you descend into it, you're looking into a different file system. Each file system has its own capacity, listed by
While you moved out
/boot, this wasn't enough to have sufficient space available on
/. Solutions: grow the
/ file system, move out more like
/var to another file system.
Are all directories under
/ mount points?
Directories not being a mount point are actual regular directories part of file system in mounted as
/. There are also special ones, which don't actually represent data on disks, but in UNIX/Linux, everything is represented as files such as sockets, kernel interfaces (
/sys), etc. and even directories itself are files.
How does this compare to Windows?
Windows tends to hide the difference between the devices/partitions and the actual file system being mounted. This means that a
C: "drive" can also be mounted as such. In Ubuntu/Linux, you'll see that a partition, e.g.
/dev/sda1 (first partition on
sda) can be mounted (content being made available to the user) to any location. There's no second root for this like Windows makes the second drive a