We wanted to upgrade an old RedHat 5.0 installation, and at the same time
use a different root partition. So, we copied whole installation from
/dev/hda2 to /dev/hdb2, sticked in a boot diskette, and tried to upgrade
over ftp (that machine did not have a CD).

Answered /dev/hda to the question which one to upgrade.

All went smoothly, until we were informed that we didn't have enough disk
space (on a 1500M partition, filled with about 900M of the old
installation). (I report the disk size calculations as a separate bug)

Tried to delete stuff we were sure wouldbe reinstalled, and nothing helped.
Turned out that the install was trying to put things on hdb2. It had
mounted /dev/hdb2 as /mnt/sysimage(?)/ and hda2 as /mnt/sysimage/a2.

In the end we Fdisk'd /dev/hdb2 into an empty partition, so that the
installer would not find it, and would be forced to upgrade hda2. This
seemed to work, but we hit the disk size problem again, and had to stop for
today. Fdisk-created hd2 to Linux again, fsck'd it, and booted back to
normal.