if the images are not corrupt then it could be a hardware or software issue.
I originally had trouble with my Seagate FreeAgent software, but updated the software to the latest version available from Seagates website. The FreeAgent software is basically a backup program, and eventually worked well, but then I found a much more flexible program called RichCopy 4.0 and it's ability to backup files is much nicer than most other programs.. and is free!
That was on WinXP and now I'm running Win7 and have only loaded RichCopy as my file backup program.
I think the FreeAgent program also had a drive analysis tool to check the external drive for failure or impending failure too.. If you loaded the software try it and see.
Just hope that the card is definitely not corrupted in any way.
You could also load it back onto the camera and make sure the images are all still intact too, by viewing the images on the camera.
I need to replace the PSU on my PC as it's failing and randomly rebooting my PC now... bye!