This has long been a problem with FTM (starting with version 2008). Typically this means that the program has run out of memory during the merge and crashes. It does not matter that CPU (and speed) or amount of memory your computer has for this problem to occur. FTM can only use (address) the first 4GB of ram and typically 60 - 75% of that RAM is use by the OS, other programs, etc..
Regards,
Regards,