In  your GMAX, go into the CUSTOMIZE menu and hit "Configure Paths".

A pop-up will appear. In the first Tab named General, you will see all basic paths including Fonts. Just choose each one and hit the Modify button to point them to where they are (mostly under your drive:gmax folder).

Also, since you mentioned I/O error, I would recommand you to do a full disk scan of your hard disks.

Excuse my ignorance, but how do you get to "customize" (in order to configure the path)  in GMAX when it won't open?

Thanks,

Pauline