As long as you have PM generate the previews when adding the files to the catalog (this is the default), you should be able to remove the disk and still be able to browse/view the images (at full zoom you'll notice that the images is actually of lesser quality than the original, of course, but still very much usable). You just won't be able to adjust the metadata of the “offline” files.
@Kirk, offline files have their green dot removed, I'd have thought it would either have stayed green (files are in a catalog) or at the very least have become e.g. yellow (to indicate file is in catalog but offline). Complete removal of the dot means (to me at least) that it is not part of (any) catalog.