Possible to verify file copies after simple copy/paste?

Hi all! I have been transferring all my old Mini-DV tapes to an external 3TB USB HD and want to copy and paste (WIN 7) to two other drives for redundancy. I prefer to continue to go the low-tech way by not using any back-up or compression programs, but still need to verify that the copies are not corrupt (I started getting a problem when I installed a USB 3 card recently, and although that card is now removed, I am not sure there is still a problem with the copying).

Is there a (free/cheap) solution to verify copies made using simple copy/paste? Any suggestions appreciated.
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Regards. Anders