cant reinstall osx from ext hard drive ... Help!

1 month ago, at startup, there was this blinking folder with question mark in it. I checked online and there was this tip to start disk utility and proceed from there. So i did it and it worked. But only for 3 days. After, I had the same problem again. I tried disk utility but this time it didn't show anything. So i tried internet recovery, which took awful lot of time but it completed. It gave me options of : either time machine or reinstall osx . i tried reinstalling but it took much awful lot of time but never got completed. So i got osx from one of my fren's mac in ext hard drive. When i plug it in, it opened, and showed all files in it. But the problem is i can't use it after unplugging the hard drive. What do i do next?