I was asking because I just got a new motherboard and new Windows install. I had left FTM running overnight to download my large tree (22,000 people) and it had rebooted for Windows updates during the night.
I restarted and it ran for 2 days and never finished media processing. I finally unlinked and deleted that tree and started over. It only took about 6 hours last night to finish the complete download and all media successfully. So, it seems that interrupting the download at any point prevents it from restarting properly or from finishing.