'Please make sure it is in the drive' while using Steam

I have been playing FM 2011 since the release date without any problems on my mac. Downloaded 11.3 when it came out and have been playing on and off since. When I bought the game I bought the disk version and registered it on steam so I don't have to have to keep the disk in my laptop. I played yesterday with no problems and attempt to start FM today via steam and when the FM application opens and gets to the home screen it just says 'Unable to find Football Manager 2011 disk, please make sure it is in the drive'. Which of course would be fine if I wasn't in Canada and my disk in Ireland! Not sure why it has decided it wants the disk today? I want to playyyyy

(I assure you this is not a pirated copy or whatever and I hope this doesn't mean I have to download some crack or anything)

Try deleting the exe(C:\Program Files (x86)\Steam\steamapps\common\football manager 2011\fm.exe), then via properties window in steam choose "verify cache integrity".

I'm using a mac, I'll see if I can find something similar I guess!

I found this same file you mentioned and deleted it, but the problem hasn't gone away :/

Something I didn't mention was that yesterday I noticed on the FM2011 website that there was transfer update, I wasn't sure if this was separate to 11.03 so i downloaded it manually. I didn't create a new game but continued with my old game anyways. Thinking back this might have something to do with it :/

Yes, patch 11.3 has that transfer update and steam automatically downloads it. Solution will be delete the entire game and redownload it (you won't lose your savegames). When you click new game you'll see you can choose between databases.

That error message would certainly suggest that you have downloaded a wrong patch.