This type of corruption is usually the result of a hardware failure. You will need to do a process of elimination to determine what caused it. Typical problems include taking the card out of the camera before it is done writing, bad card, bad card reader, etc.
Eric, thank you for taking the time.
OK, so after testing I'm even more confused. I thought I isolated the issue down to my iMac's SD reader because..
-I've had corrupt files come from my 60D and my 6D so can't be camera
-I've had corrupt files come from 6 or so different SD cards so can't be an SD card issue
-Files importing via my Macbook Pro imported fine.
I was sure it was a hardware issue on my iMac UNTIL images I just imported onto externals via my Macbook pro SD slot have this corruption—probably 1 every 50-100 photos.
Now I'm very confused because it seemed a sure thing it was my iMac but now I don't know what to think. Other details to consider..
-This corruption problem happens in multiple catalogs
-The raw file displays the correct image at the thumb level but when blown up in Quick Preview, displays as fully black
-One of my corrupt previews turned from a black box back into the proper, un-corrupt image. How is that possible??
-When I delete the files and reimport, about 70% of the time it imports without this issue. the other 30% requires another re-import because it repeats the corruption.
-It's rarely two error images in a row but sometimes they happen closer together and sometimes randomly spread throughout
-The most recent corruption via my MacBook was using a brand new 128GB SD
I have used the SD on the iMac once before though. Is it possible the iMac SD slot is messing with the cards and hence allowing for my brand new Macbook to have issues importing now also?
I'm greatly confused and would love any help I could get. Hopefully this thread can help future me's also