Comment 2 for bug 160649

1. Run installer, give mount points to non-install partitions. After install, reformat those other partitions but leave Ubuntu ones alone.
2. Expect to be able to boot Ubuntu, since its data are there and intact.
3. Ubuntu goes into rescue mode when it couldn't mount something at boot - even if it doesn't and won't ever need anything from those mount points to continue normally.

I haven't tried it in a while, so maybe it's fixed, but I'm pretty sure the first post had all the steps, the expected output, and the actual output.