It's because you have been logging in with a password on your external hard drive when it was in your other PC. When you are using a password you have protected others from seeing the files so that only you can access your files when you are logged in. Seeing as you aren't logging into that windows installation you won't be able to access the files. Put the hard drive back into your old PC - move all important stuff out of that folder and you'll be able to copy it without problems.
Just plugging the hard drive in via IDE wouldn't solve the problem, but is always better than using USB.