I tried almost 2 days almost everything to save the game, just wanted to post all the possibilities which didn't work and believe me, I tried quite a lot. But right now I found the solution. I hope it'll work for all the other guys with the same problem here too.
First: I had this message "Unable to read drive" too, not just on top of the save-dialog but also in the load-dialog.
I could load all the savegame which were included in the zip-archive but there was no save of mine. Neither was there an autosave-file although I checked the options and I never got an error message from the system (no message box at least). Same happened while saving games. There was that message "Unable to read drive" but saving seemed to work.
OK let's stop the palaver: I just realized that there is a small scrollbar on the right side of the dialogs... just scroll down and you'll find all of your savegames and also the autosave.
I know it sounds unbelievable but trust me, it happened right that way to me. I haven't seen this damn scrollbar for 2 days... I was just irritated from the message "Unable to read drive". Probably I thought I couldn't save because (like most of the people here now) I'm used to see the last save on top. But that does not happen at TTD.
btw: The latest patch 2.5 beta 6 from June 16, 2006 at
http://ttdpatch.net/src/ works fine with the version you can download here. You only have to start with the ttdpatchw.exe then.