This question already has an answer here:
My Macbook Pro ran into a problem and reported Error: -50. I cannot open applications such as Chrome, Word, and others. I cannot download content or move content to the trash because it says the disk is full, even though it still shows 80Gb of free storage. The Ram is free and I cannot run ClamXav to see if it is a virus. Disk Utility verifies the disk but the permissions have all changed and cannot be repaired. Some permissions errors include "user differs on private/var/at, should be 0 user is 1"as well as various others. I cant even see/open external drives I connect. Any idea how to solve this problem?