Hi,
If you formatted and the problem went away until you connected your external. I think that we have found the problem area. I would recommend that you scrub your external hdd clean, with a program like
GParted.
GParted is an Open-source program that is run from a livecd or usb flash drive. This is a great program for doing re-formats.
After that if you are still having issues then I would take a look at the link posted above.
Cheers!