Comment 2 for bug 199048

You have to disable swap on a device (and in general unmount everything on it) in order to convince the kernel to reload the partition table if you make any changes to it. If you don't do this, partitioning can get very confused.

I do agree that there is room for optimisation here. Swap only needs to be disabled on devices that have actually changed at the partition table level. This would allow the sort of multiple-disk case you're talking about, as well as installing onto a pre-partitioned disk with low memory.