Boot to recovery console, and try running chkdsk /r. When you finish, go back to windows and try again. If it still persists in restarting, unplug EVERYTHING except the bare essentials (if you have onboard graphics you can use, even unplug your graphics card!!!) and try again. If it's STILL doing it, Take a look at your internal hard disk (the external one shouldn't be an issue as both your USB headers and your external HD should be unplugged!), find out what its make and model is, and download the diagnostic utility for it from the manufacturer - if you can't find the HD diaagnostic utility, tell us the make and model and we'll post a link.
A different approach here, but for some reason I'm gravitating towards a hard drive/partition/corruption issue.