iTunes Transfer Problem

I have a macbook pro. When i try and transfer my songs from iTunes to my 8GB usb drive, the screen kind of gets taken over by a grey screen with a light opacity and then my laptop completely freezes and there is a small dark grey box in the middle saying "You need to restart your computer. To restart hold down the power button"

*When i transfer about 80-100 songs it works okay, but its usually just the big transfers*

This has happened several times and I am just wondering if anyone had any possible idea as to what is causing this.

Note: I have tried different USB's so its not that

This is what the screen looks like...