I have the very same problem when any USB hard drives (or for that matter, any USB device that takes any advantage at all whatsoever of anything above USB 1.x performance) are in use with any of my systems: Every single system of mine slows down in a similar fashion when I tried to use the computer at the same time anything that requires anywhere close to full USB 2.0 High-Speed bandwidth is being transferred through the USB interface (this means that any file transfers through USB or any use of transcoding programs with video files located on any USB external drive will take up the entire bandwidth of the interface or the USB device itself, whichever is more restrictive). Face it, the USB 2.0 High-Speed interface is simply nowhere near fast enough to keep up with the performance of modern hard drives (remember, these 1TB drives are capable of maximum sustained physical transfer speeds of over 90 MB/s, but the USB interface is restricting it to barely over 30 MB/s), and it uses up a lot more of your system processor's load (aka "CPU Utilization") than it should have.
In other words, it's the entire USB 2.0 High-Speed interface on any system that's to blame (and this slowdown of system performance during USB data transfers occurs no matter which brand of external hard drive you use). If what you've been doing (running cryptload) is what you intend with an external hard drive, you will need to purchase an eSATA card (unless your system's motherboard has built-in eSATA ports in the rear) and an external hard drive with an eSATA interface or an internal SATA hard drive and eSATA external enclosure combo. Too bad Western Digital currently does not offer a Windows-compatible external hard drive kit with anything other than USB (since they discontinued all of their eSATA and FireWire external hard drive kits for Windows last year) - and their recently-introduced USB 3.0 drives cost way more money than other external hard drives of the same capacity (since current SATA-to USB 3.0 bridges/translators/converters are astronomically expensive). And a USB 3.0 external hard drive kit is out of consideration (unless you already have a USB 3.0 controller in your system) because it would have defaulted to the USB 2.0-compatible mode on your system and thus you would have had the very same problem with slowdowns.