sounds like the external hdd is either drawing to much amperage from usb, or the data transfer of the drive is keeping the usb hub too busy for the mouse to get its data processed in a timely manner.
Try using the external hdd on opposite sides of the laptop if possible by laptop design.
Does this happen with normal usb flash drives? if not does it happen while moving a large file to flash drive? if yes then its the usb hub going out. if no then you can move to the external hdd next.
does the external hdd produce the same result on another pc, (plug same mouse and external hdd into two closest usb ports on another machine and test) if the same results happens here then it is definitely the the external hdd. you could try taking the hdd out of the case and just plug it into a desktop pc to see if it is the drive that is malfunctioning or the external case enclosure.
If it is the external enclosure that is bad you may be able to get a replacement from the manufacture or buy a third party from http://www.newegg.com or http://www.ebay.com
If the result is that your usb hub is going out, do you have a pcmcia slot on your pc. you can get a usb 2.0 possibly a 3.0 adapter card that would eliminate this problem