I have my downloads on 2 1 TB external hard drives. My primary drive is plugged into my DVD player and I use it everyday. My secondary drive is for back up purposes just in case the first drive fails or a file becomes corrupted. I just switched the back up to an SSD drive. I almost always buy both DVDs and downloads. My Cathe download library is > 150 GB. The hard drives will work for a long time even now that I started to buy Live downloads.
Cathe's download format is M4V which is the same as mp4 for all practical purposes. Windows media player plays them without any problem. Most DVD players and TVs can play them directly from a USB drive. If you are having a problem playing them, make sure that the files have the right extension (M4V). If you are still having problems, contact customer service. They are always very helpful.