I've had this happen a couple times before, but what fixed it always for me was to change the permissions of the hard drive itself and be sure you have UAC disabled. First things first, make sure that the UAC is turned off (in the User Account section of the Control Panel) and restart the computer.
Then, open My Computer and locate the external hard drive that has all the source files you want to copy. Right click on this drive and select properties. In this window, select the Security tab, and then click the Advanced button at the bottom. This will now bring up another window, which you will click on the Owner tab. Click the Edit... button at the bottom and you should now see a list of users or groups listed here. If your current username is listed, just double click on your user name in the list. If not, click the "Other users or groups..." button and type your username into the field here and press enter.
You will have to close out of all the windows, then, for these changes to apply and depending upon the amount of data there on that drive it might take a while to apply these changes. Some system files may not allow changing this property, but I believe you should be able to allow Windows to skip over those files if a warning message pops up about those files.
Then you can open up this same properties window, go back in to view the owner of the folder, and should see your user name listed as the owner. At that point you should have complete access. If not you may have one last setting to apply:
If you still come up with user account access permission errors, then again open the properties of the hard drive and select the Security tab. Click the Edit... button to open up a new window. Here click the Add... button, and then enter your user name into the field and press enter. You should now see your user name appear in the list of group or user names along with standard accounts such as Authenticated Users, System, etc. Click on your user name, and below that click to select ALLOW on the "Full Control" option and it should highlight and select all others below as well. Then just click OK.
If nothing else fails, then yes you might have to use a live boot CD from Linux, which yes is more complex to do but should be able to accomplish it. It just becomes confusing and more difficult because you have to know how to use linux commands to mount your drives, and hope that your live CD also has drivers for your hardware or it probably won't run.
