World won't start

[ferrettomato]

I have made a really big world in Hexxit (my vast search for treasure has brought it up to 503MB) that also has some nice builds of mine in it. The other day when I brought it home from a friend's house ( I take it there on a USB drive) I put it back into my saves folder and attempted to load it. It was stuck on "Loading world" for as long as my patience lasted, and then I closed the window with minimal hassle. I tried deleting it and putting another copy of the world on my USB drive in, but when I try to delete the world it says that another program is using it. I cannot find a program that is using it. What's wrong?