I was trying an upgrade from 4.2 to 6.1.92. After choosing the Upgrade
option, in the Upgrade Examine screen, I choose 'Customize packages to be
upgraded', and then 'Next'.

On this machine, hda3 is /. When I click 'Next', 'mount' on VT2 shows hda3
mounted for a few seconds as /tmp/sysimage and then it is unmounted.

The installer says it's "Reading package information" and then displays the
Individual Package Selection screen. I choose 'Back'.

On VT2, 'mount' still shows / (hda3) unmounted.

I click 'Next'. I get "Error mounting hda3: Device or resource busy". VT2
still shows hda3 as unmounted.

I know this is less than clear, let me know if I need to clarify.