but including retro pie I think you might just make with exactly 16 GB
@kalimah You say you tried "a couple of ways" but don't detail these. Please provide more information. There is only one reliable way to manually install RetroPie on top of Raspbian and that is the official way. Please see docs
@edmaul69 It uses 550mA, other 2.5 harddisks use the same, the max an USB 2.0 port can use is 500mA. So it is indeed just the same drive just as any other 2.5' drive.
From what I've understood people got problems with external harddisks because of their power supplies, they didn't provide enough juice.
Thanks for your assistance. From the link you gave I used FAT32formatter, it appears to have worked. I got some error messages from the program, but it seems to be working ok.
@infernusamongus hey did u fix the problem did u expand ur external hdd