I had Windows 10 build 10240 running on my older computer but the upgrade to build 10586 failed.
This is an older system - non-UEFI - with a MBR boot volume.
The boot volume has 4 partitions: Windows 10 Pro (NTFS), Data (NTFS), Linux (ext4) and Linux swap. There are none of the extra 'normal' special partitions like the small EFI and MSR one usually finds on more modern systems.
The system boots from grub to either windows or Linux.
This has worked on Windows 8 and the upgrade to Windows 10 build 10240.
But when I install the newest Windows build 10586, the new version of Windows 10 destroys the grub loader and I get an MBR load error.
The volumes seem to be OK, but the boot info gets trashed.
I've restored back to Windows 10240 and tried on a different hard disk and I got the same results.
I tried restoring the MBR and also tried using 'bootrec /fixmbr' but nothing seems to work.
I am restoring a third time back to Windows 10 build 10240 and I will try something else.
Perhap I should try to remove grub before the next upgrade attempt.
Has anyone else encountered this issue and have any ideas?