How do Mac file imports from SD cards work when the option is set to delete after import? My uncle recently imported files from his SD card and there were serious errors. I have no idea WHY that's occurred, but the videos I took are once in a lifetime. I shot on .MOV format, but the card also contained some other Nikon camera format. 

Images taken on both the Nikon and the Canon were imported. But he says that there was some problem with the videos. He mentioned "convert" and "mpeg4". I asked him what the net size after import was, and he said 20 MB. That's impossible even if conversion occurred because the videos are, in their basic format, at least a few gigabytes. 

So did my Mac import, then delete those SPECIFIC files or did it format the whole thing?
There are many programs that will import photos from a memory card, so you might want to get more information so that people don't have to guess.

Image Capture imports images and then once they are verified - deletes only the files. This deletion is easily reversed with run of the mill "camera card software" that recovers deleted files. See https://www.prosofteng.com/picture-rescue-2/ for one example of that.

The other Apple software generally works the same (iMovie, iPhoto) - but if these photos are really irreplaceable - you could get a professional quote to recover the files and interview your uncle to understand what he did or look over the Mac to see what software is installed and rule out something that would reformat the drive.