Unable to recover files using Recuva

I am having a difficulty in recovering some images that I just accidentally deleted from my external hard drive. I immediately used Recuva's deep scan option and it displayed all the images in an excellent recovery status. The problem is the continuous error message I get when I try to click the recover button. The message says "the volume for a file has been externally altered so that the opened file is no longer valid". Please help. How do I solve this?

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