For example: I have an external HDD and wish to mount the "backup data" partition so that it appears as a separate device, just for coolness.
mkdir -p /mnt/disk
mount -o loop /dev/sda3 /mnt/disk
/dev/sda3 on /tmp/mnt type ext3 (rw,loop=/dev/loop0)
/dev/loop0: [XXXX]:YYYY (/dev/sda3)
3 years ago