External HDD files truncated on transfer

I've experience similar problems copying to an external drive and it can be frustrating. First thing I suggest everyone do is grab something like Teracopy or Supercopier or Ultracopier to replace the default Windows copy. All of these have a verify on copy option. If you enable that, immediately after copying the file your computer will read it back to verify (via checksums) that the copy matches the original. That means the copy will take twice as long, but you'll know immediately if the copy was good. You won't have to watch the entire video to spot any copy errors.

When I've experienced this problem, a couple times switching to a different USB cable fixed it. I usually use a retractable USB cable but it seems to sometimes give errors. I switched to a regular cable and things copied fine. One time after a lot of troubleshooting I tracked it down to the external drive enclosure. It worked fine before but was getting a bit old, and apparently isn't reliable anymore. Other things to try are plugging the drive into a different USB port. Or the two-headed USB power cable which ships with some enclosures. It's possible the drive is trying to pull a little more power than a single USB port can provide, which is causing write failures. This cable will let the drive draw power from two USB ports.

It's possible the drive itself is failing. The easiest way to check would be viewing the drive's SMART logs. Unfortunately you can't do that over USB on an external drive. You have to remove the drive from the enclosure and plug it into a SATA port on a desktop. (And this isn't even possible with WD's Passport drives because they don't have SATA ports internally.) Check with the manufacturer if they have a utility to do this, because opening it up probably voids the warranty. Toshiba has a pretty generous warranty on their external drives so if the drive is at fault, replacing it should be a snap.

The other possible cause is that your system has a virus which is trying to insert itself into files you copy. Try copying the video to a different external drive or a flash drive and see if you encounter the same problem.