Moving photos to and from PC--not able to copy

I am trying to move photos from my PC to my phone and vice versa. My phone,is recognized when I select MTP mode. Sometimes it can find the photos and sometimes it can't. Whenever I can see the files, and try to copy them over, it says "Cannot copy Img. The device has either stopped responding or has been disconnected." Why is it giving me this message? I havr my photos on an SD card, but it says the same thing when I try to copy to or from Internal Storage.

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