It's not recommended, but I use my old 60GB SSD without external power supply (needs around 2W).
The SSD still has 100% in CrystalDiskInfo after 3 years in a Netbook and 6 months in the Wii U. Still less than 1 TB written. Playing games almost doesn't write new data, but it reads all the time. I'm backing up my games every few weeks.
I know a SSD is overkill with USB 2.0, but ZombiU and has shorter loading times and no more visible texture pop ins (they are noticeable when you load the game from disk, internal memory, external HDD or external USB. Yes, I tested everything...). I'm not sure why a SSD is better than a external USB stick that is much faster than USB 2.0. Are the access times that different?
Man, there should really be an option to install disk-based games, some third-party games suffer from streaming issues. Loading the data from disk + internal/external memory simultaneously would solve some problems.
