If you go to Tools-Replication you can synchronise now (which should stop the error message) and also recover design master to find or return to the original. Somehow your DB has become replicated. Replicating a database allows 2 copies to be used independantly and then synchronised at a later date, but the replication process adds fields to your tables for unique identifiers and the whole thing gets quite complicated and difficult to change back. I wonder if you managed to create a replica thinking you were copying the database for back-up purposes? - just a thought. Good luck with it and post again if you need any more input.
I have made a copy of the database and tried to make it the Design Master and tried The Tools > Replication > Recover Design Master > Design Master Deleted > Make Design Master > It says it needs to close then I still keep getting this Exceeded the number of days message.
I am pretty certain the replication occurred when I first saved the database after I updated from Office ’97 to Office XP. I suddenly found that I had an extra copy and deleted the wrong one. It appears to be an Access 2000 file.