A large chunk of unexplained high entropy data is a bit of a giveaway.

You can't really hide the fact that you're using encryption, because it doesn't really look like anything else apart from random data & people don't usually go around piping /dev/random into large files for no good reason. Compressed data is high entropy, but can be decompressed to something lower entropy, so you can easily eliminate png filess, zipped files etc.

I thought full disk encryption left the drive looking entirely unformated. Although you'd probably have truecrypt installed on the computer. You would also want to not save volume mounting history.

But if you had truecrypt installed and 1 drive that wasn't formatted... Yea