Unerasable White Squares Appearing in My Photoshop Files

[JuliusT]

Hey all,

I am using Photoshop CS6 and files that I have spent 100s of hours working on with no problem suddenly have unerasable white squares appearing that I cannot get rid of. They seem to be attached to specific layers, so I can delete the layers and they will go away, but I can't lose this work. Here is a photo of the squares below (the red and black are supposed to be there, the white squares are not). If it helps, I am on Mac and I have sent these files to another computer and they also manifest the unerasable squares.

Any help is appreciated!