Thanks for the reply. I had read a similar posting about the 4GB limitation after I posted my message, and my FTM files were sitting on 3.91GB which might explain it. Of course 1.3Gb of that were the two backup files that FTM recommended I create before I commenced the merge. I have since copied the backup files to an external disk to create space, compacted my files and will try the merge again.
It is a bit frustrating that I followed the program's instructions to the letter, including the backup files, and was never warned it might crash. Indeed, why can't the program warn about that at the beginning of the merge rather than an hour into the operation?