In most cases, you'll want to use an external hard drive to transfer files from a PC to a Mac. Fortunately, using an external hard drive to move files is easy.
Windows Explorer or 'My Computer'? If so you could make use of them to drag and drop the files from one drive to another. You could also make good use of the Command Prompt (Click Start then Run, type in CMD and click OK) as long as you know the drive letters of both drives (which can be found with Windows Explorer). The command to use in that command prompt window would be - XCOPY E:.
F: /S /E /H /K /O Where E: is the drive letter of the drive containing the files to be copied and F: is the drive you want the files copied to. On your system the drive letters will probably be different. If I understand you correctly you still have the originals on your C drive, (hard drive in the computer), and have 'copied' these to that first external hard drive. If that's right, then I would just do the same again.
Copy the files from the computer's hard drive to the new, 2nd external hard drive. The other ways get messy, eg; 1 Connect both USB hard drives to the computer and copy from the old to the new. 2 Connect the first external hard drive to the computer, copy the files back to computer's internal hard drive. Disconnect that first external, connect the 2nd, new external hard drive, copy those files to the new one. Test to see they are all there, then delete them from the internal, (since you already have them on there). Windows Explorer is the easiest tool to use in all cases. You can move them the same way you got them to the other drive in the first place.
Copy & paste. Do keep in mind that you will be moving a large amount of information, unless you do them one at a time. Once your additional drive is ready, click on a few files & drag them over to the new drive.
Being that it's another drive, it will copy them, not move them. I do this all of the time with the main drive back up. Be sure to let the drive stop blinking for a moment, before going for the next gig or two batch. Usually that will be the time you, yourself are ready to click on the next ones. A warning of course, something may choke, so only do a few files at a time. Don't have other apps running, updating & downloading while you do this, one of them may cause a glitch, & you think you have a file, when you don't. Overall, this is very easy & you can have many copies of the same file on many different drives.
I usually just click a dvd's space of data & drag it to the next drive, spot checking every few files to be sure. Once done, restart the computer & check a couple files again.