On my external hard drive, many files have randomly appeared that begin with ._ have appeared, and are only viewable with hidden files set to appear. The ._ have the same file type as other files in the same folders, and the names are similar to files in the same folder except with the ._ in the beginning, and all of them are approx. 4-6kB. They don't appear on all files, but they do show up on more than half. What is the purpose of these files and is it ok to delete them?