I had a similar problem in 10.0.4 where I could never mount dmg files in diskcopy for some unknown reason it always turned up an error. I remember hearing other people with the same problem, and I think it was a known issue between os 9.2.1, classic and 10.0.4 depending on where the files were downloaded.
You have two options. Sometimes toast can mount images that disk copy cannot. Simply drag the disk image onto toast and it will give you options if successful. I like mounting images via stuffit menubar the best, since it never verifies the image. They mount much quicker.
BTW, i've downloaded my share of crap from hotline and carracho, but find that seriously hacked image files are rare, very rare. Most things should mount from one of these programs, if not everything.
Luck,
Seamus