> Converted GPT to MBR/EMBR.
> To upgrade Win8.1 only, and avoid any HD or partition corruption, is any
> precaution needed to assure Win10 will upgrade only Win8.1?
If you want a multiboot system, the following should work and preserve your existing partitions; as always, BACKUP your system first, in case it all goes horribly wrong. Procedure:
(1) Unlimit primaries in your BIBM settings.
(2) Create an EMBR for each internal disk in your system.
(3) For the disk that is the target for the installation, create the partition to which you want to install Windows (e.g. 50 GiB), plus three tiny partitions at the end of the disk (e.g. 1 MiB each). Call these latter partitions FILLER1, FILLER2, and FILLER3. In your case, you want to update an existing Windows 8.1 partition, so that is the partition to use.
(4) Create a boot item that specifies WINDOWS, FILLER1, FILLER2, and FILLER3 as the partitions to include in the MBR for that disk. If your Windows 8.1 installation already uses one or more extra partitions, include those, and then fill up the remainder of the MBR with the FILLER partitions. Maintain whatever partition order already exists for Windows 8.1 and its extra partitions.
(5) For other internal disks in your system (if you have any), create four tiny 1 MiB partitions at the end of each disk. Call them FILLER1, ..., FILLER4. Place these in the MBR for that disk in the Windows boot item.
(6) Now start the installation of Windows, and when asked, select the 50 GiB partition as the target for the installation. In your case, Windows will likely pre-select the Windows 8.1 partition.
(7) At the end of installation, you can remove the FILLER partitions from the MBR in the boot item, and replace them with your data partitions. However, don't delete the FILLER partitions; leave them on your disks in case you need them again in the future.
The reason for this rigmarole is the following:
(a) If Windows sees any unused space on either the target disk or other internal disks, Windows will use it to create extra partitions, possibly overwriting your data in partitions that Windows cannot see.
(b) If Windows does not see any unused space, it will shrink the largest partition in order to create some unused space that it can use for the extra partitions.
(c) The only way to stop Windows doing this is to ensure that there are no unused entries in the MBR. Windows does not delete existing partitions. Hence, if there are no unused entries in the MBR, there is nowhere for Windows to place the extra partitions, so it won't create them. So no risk of overwriting the other partitions that Windows cannot see.
So you need three FILLER partitions on the target disk, and four FILLER partitions on each of the other internal disks. This procedure has worked for me in the past (Vista several years ago, Windows 7 last week), but there are many variations possible, so if I have omitted a vital step, please post a correction!
-- from CyberSimian in the UK
Edit: Changed size of Windows partition from "50 MiB" to "50 GiB". The FILLER partitions can indeed be 1 MiB in size.
Last edited by CyberSimian
on Sun Aug 14, 2016 9:32 am, edited 2 times in total.