USB Analog Video Capture Tips
I'm importing old VHS movies to my hard drive using a Plextor USB ConvertX. The hard drive where I'm storing the video is also external USB. Should I anticipate any problems during video capture?
In the world of USB 1.1 it was easy to have too many devices connected to your USB controller, with printers, Webcams, scanners, mice, keyboards, hard drives and a whole host of other peripherals all vying for available USB 1.1 bus speed of 12mbps. Over the past several years, most new computers ship with USB 2.0, which has a 480mbps bus speed (about 40 times faster). In most cases, including a scenario you describe with both hard drive and analog video capture using the USB simultaneously, there should be plenty of USB bandwidth available. If you're unsure how to find out what type of USB controller is in your PC, see my guide on checking for USB 2.0. There are several things you can do to help insure an errorless transfer (not all related to USB).
One key thing I recommend in doing video transfer over USB is to disconnect unused peripherals. If you have a Webcam, scanner, printer, etc connected, disconnect them while doing the video transfer. If you are stuck using USB 1.1, this can be vital to your data transfer, It's likely these peripherals won't have any impact on USB 2.0 transfer, but erring on the safe side never hurts.
If the external hard drive has both USB and FireWire connections (many do) use the FireWire connection on the hard drive instead of USB. Since the video transfer is happening over USB, you'll insure more available transfer bandwidth at both the import and writing to hard drive phases of video capture.
Consult my list of 8 ways to prevent dropped frames during video capture for additional ways to help improve your video capture results. The list was compiled specifically for capturing video from DV camcorders, but most of the advice is also applicable to analog video capture.