Comment 2 for bug 1533587

To be precise, you have not shown that prep is presented as an option in either case. If the guided partitioning is used, there will be a partition set up for prep; and when displaying details about this partition its usage will be shown as "prep". But if you try to change the usage, is "prep" given as an option there? I suspect that it won't be.

This points to a problem with choose_method/prep/choices from partman-prep, either not being invoked correctly, or not correctly outputting the description string.