What about backups??? Do you have them. Your question implies no?

First any method you use for large numbers of files has issues.

Move.... potential issue. As you move the folders to another drive inside or outside the computer, if the receiving hard drive fails (it happens) you lose the pictures that moved to the now failed hard drive. If the source hard drive fails, you lose the pictures that have not yet moved, plus your C:\ drive. Either way, lost images. I raise this question because a very large move like this stresses the drives and failure can take place. Again lost images, or lost everything except the files that made it across.

Copy, then go back after affirming files are on receiving drive, and delete the files you intended to move from the source drive. Failure of either drive (again stressed) results in lost images only if the source drive fails part way through the process. Safer but....

It would be nice to know if you actually have a backup of your images, or if you mistakenly consider this moving of files is in lieu of a true backup.

A true backup is having your files redundant on two locations at all times. Therefore, I would not simply move files from one drive to another and consider that a backup. Also, I would never do such a move or copy unless ALL of those files existed on other media.

By other media, I mean ONLY another external drive. CD and DVD disks are not archival for any guaranteed length of time, and in the classes I teach, neither jump drives, thumb drives, or CD/DVD qualify as backup media. If you are not aware of the terms CD Rot and DVD Rot, look them up on Yahoo Search or Google.

So, the pecking order should be this:

1) Another complete set of your imaged or data on an external drive that is NOT used as a working drive. That's your backup.
2) Then do the copy, check and delete, for a working external drive (yes, another external, I have a client who has 8 such drives, plus another 8 for his back drives which are stored)
3) Always move files you are editing to the drive in the computer. Photoshop and other editing programs become very slow when they have to transport commands and writes through the USB system (unless you have USB 3.0 the new one involving fairly new computers and switching devices to USB 3.0 capable)
4) Bottom line is that editing programs work best on a drive inside the computer that's hooked into the interfaces on the motherboard. Laptops generally preclude this.

25 years of computer consulting and teching here. Believe me when I say drives do fail during these processes, so stay away from move commands. In fact, my experience with hard drives over those years has been that the drives that fail will more often be low time drives, so be wary of failure of the NEW external drive.

And, be double redundant on your important files for a true backup.

OTOH, maybe it won't be such a big deal to reschedule and reshoot those:

Special Events
Travel Home and Abroad
Children growing up
Grandma getting older
Recreation
Weddings
Even perhaps all your professional photog jobs
and on and on.