External drive's connectivity is USB 3.0, therefore the only explanation that could be plausible is that those two peripherals interact with each other and cause a conflict. Try using one or another and check the behavior, and if indeed it confirms my suspicions, insert one of them into another USB port and install the USB 3.0 drivers for your PC (usually found on the CD-ROM of your motherboard).
Secondly, try using other USB cables as well since they're fragile and get broken easily, then repeat the procedure.
