Note that it depends on the file system involved and what else is going on on that particular drive (or partition).
MS filesystems like FAT and NTFS make it fairly easy to recover files in some circumstances.
Unix-related filesystems, OTOH, are usually much harder. This includes Macs and many other Apple products. With iPods, you can elect which filesystem to use in some (all?) cases. If you select FAT and connect your iPod to a MS-Windows OS PC, then life is easier if you want to recover data off a messed up iPod.
Note, though, that the OS is frequently updating things on it’s drive/partition, so any data you want to recover on the OS drive gets quickly harder to recover. One reason many people (like me) like to have separate partitions for the OS and the data.
Plug in devices like cameras and mp3 players usually don’t have any background file changes going on, so they’re easier to recover.
OTOH, if there are pictures on the camera you deliberately want to obliterate, you need to fill the card up with new stuff. If it can do video, start it recording and let it go for a while. Or you can copy junk files off your PC to the camera if it will let you.