As mentioned previously, I've not figured out how to support Mass Storage Class devices yet so only the first LUN will be recognised.
Sorry about that, its fixed in today's build.
It probably depends on how the FAT partition was opened. When a FAT32FS mounted partition is deleted, PM will attempt to dismount it - but only if it was previously mounted by FAT32FS before PM was loaded.
FAT mounted via an Image filesystem through a linked file will still appear to work if the linked file isn't deleted (ie !Boot.Loader).
FAT drives mounted via an Image filesystem might also still work as PM can't determine the drive is mounted, so can't notify the filesystem that it's been deleted.