0, 10 and legacy are related to multi user feature. 0 is supposed to be your main user, while 10 is your secondary (that's why it's a full new folder) and legacy is supposed to be like the old /sdcard. When this happened to me I simply flashed a ROM and then moved the 0 folder back to /sdcard. Then checked if there was anything important in 10 and afterwards deleted it.
I'd suggest you installing TWRP recovery, it never failed on me
"Time is too short to cry, long enough to try." - March of Time (Helloween)
Sent from my KitKat Nexus 10