slotMusic/slotRadio has tried to sell music through SD cards. They never took off, but a full SD card would probably be the best balance in terms of reliability, size, and capacity in my opinion. Plus, a surprising amount of media devices still carry SD card readers.

Flash storage developments came too late, but the spinning disks and risk of physical damage to the plastic are an annoying risk to have to deal with. Even Minidisc still required some form of physical motion.