I finally finished playing with my new Lenovo laptop, and decided that it was time to perform a clean Windows install to use as my "production" system (to replace my ageing Sony).
I deleted all of the partitions, re-installed Windows 10 from the Recovery USB drive, converted from GPT to MBR, deleted the "EFI" and "Microsoft Reserved" partitions, installed BIBM, converted from MBR to EMBR, and performed my Windows customisations. I then connected the laptop to the internet, and tried "Windows Update".
Initially it claimed that Windows was up to date, but on the third or fourth attempt it started downloading the 2016 Anniversary Update. I had "Task Manager" open and could monitor the speed of the wired connection to the ADSL modem. I got a consistent 16.3 Mbit/sec, which I think is reasonable (my broadband line is not superfast -- probably a 20 or 24 Mbit/sec limit). The download took a long time, followed by another long time for installation to complete. Finally, it rebooted...
BIBM started and immediately claimed that the partition table had changed, and did I want to accept the change? I selected "Yes". In "Partition Management" I could see that the Anniversary Update had created a new WINRE partition.
Now the disk is 1 TB, but I had not yet resized the partitions, so there was only about 250 MB of free space where the "EFI" and "MS Reserved" partitions had been. That was too little for the new WINRE partition, so Windows had reduced the size of the 950 GB Windows partition, and created a new 450 MB WINRE partition. I was lucky -- very very lucky.
If I had had several operating systems installed on the disk, Windows would have seen several large areas of apparently free space where these other operating systems resided, and I have a strong suspicion that Windows would have used some of that free space for the new WINRE partition, thereby corrupting one or two of the other operating systems. It did not do that because the existing free space was too small.
The mistake that I made was that I did not follow my own advice, as detailed in this post:viewtopic.php?f=2&t=2302#p12098
I had left two of the MBR entries unoccupied, so Windows was able to create the new WINRE partition and place it in one of the empty MBR entries. If I had followed my own advice and filled those two unused MBR entries with dummy placeholder partitions, Windows would have been prevented from creating a new WINRE partition. Either the Anniversay Update would have failed, or Windows would have skipped the WINRE part of the update. In either case, overwriting of parts of the disk that Windows thought were unused would have been prevented.
I like EMBRs, because they provide a flexibility to my use of the laptop that I value. But EMBRs are not without risk, and to eliminate the overwriting of apparently unused parts of the disk, you should always FILL the MBR, so that there is nowhere for an OS-install or OS-Update to place any new partitions that it decides to create.
-- from CyberSimian in the UK