When you created the backup to Windows copy, it makes you run a verify on the file.  Did that verify come back clean, or did it mention any problems?  You can also run a Verify on the file, just go to File > Utilities > Verify data.  If that comes back without any errors, you might want to check the QuickBooks.log file to see if anything is reported in there for problems (a C=44 is usually a problem with a transaction or multiple transactions).  Depending on if it finds any problems in the log file, those problems will need to be addressed first before you can create the backup to Windows copy.  The log file is located at the following location:

Macintosh HD\Users\(user name)\Library\Logs\QuickBooks\QuickBooks.log

If you do not see the Library folder (it may be hidden by the system), you can access it by going to the Finder and then select the Go menu and choose Go to Folder...  In there, type /Users/(your user name)/Library and it will take you to the folder, then you can navigate to the Log file and open it.