There is very little to go wrong with them. I have tried the Seagate GoFlex 500GB with no external power supply and it worked perfectly. I also used a powered Lacie and no problems. I do not think that you will get too much grieve with this. The sizes should be between 32GB and 2TB.
Let me clarify, I have used this for recording which once you registered the HDD with the TV then you will not be able to use the HDD for something else. They can also be used for playing media files like mp3, mp4, jpg but in this case you cannot registered the HDD with the TV for recording and it should be formatted with FAT(12,16,32.ex) For media file playing you could use smaller sizes as well. It also does not support GPT formatted disks they should be MBR.