I am running Mac10.10.5 on a mac book pro. I purchased and was successful in running Endless Legend on the Steam platform. However, now the guardian expansion pack will not download or update and I now get a "disk read error" report. I can now not run even the old version of endless legend. Effectively I cannot use the game. This problem does not exist with my other Steam games. What is the solution.

Did you try uninstalling the game completely from Steam ("Delete Local Content ..."), then installing it again ?

I'm on Windows, so I don't know what disk checking utilities are on OSX, but a quick search brought this one up : https://support.apple.com/en-us/HT201639