In my experience, before running 'SYNC' the help desk advises running the "compact file" command off the drop-down menu "TOOLS". This may be essential to keep SYNC from crashing in mid-process.
Instructions below per the FTM Companion Guide, page 226:
"As you work in your trees you will add and delete quite a bit of data. However, even when data has been removed from a tree, the file may still be large. You should compress your tree files periodically to optimize performance, remove unnecessary items, and reindex the file.
1. Click Tools>Compact File. The Compact File window opens.
2. If you want to back up your file before you compress it, click the Back Up File Before Compacting checkbox (recommended).
3. Click Compact. If you have chosen to back up your file, the Backup window opens. Change any options as necessary and click OK.
4. When Family Tree Maker is finished, a message shows how much the file size was reduced. Click OK.
Because file compression happens behind the scenes, you won't see any changes to your tree, but you should notice better performance and a smaller overall file size."