I've been dual booting Windows 7 Pro & Windows 10 Pro (Build 1809 to be precise) for quite a while now.
The aim was to see if i could use 3rd party software and run Window 10 with my existing software and tweak the user interface so it resembled as close as possible my current Window 7 way of working... e.g. without ALL the bells and whistles that the are enabled by Windows 10 etc.
The aim was never to do get Windows 10 into a state where i could simply delete my Windows 7 partition, it was more a proof of concept test to see if it could be done.
For that reason i wasn't really interested in 'upgrading' Windows 10 as later Builds were released... the horror stories i've read about some of the problems later builds have caused, that was even more reason to leave upgrading Win10 alone until i was sure the concept could work the way i wanted.
The alternative was going to mean moving to Linux or some other operating system.
I also didn't install Windows 10 onto a seperate Drive, i chose instead to partition my SSD drive and have Win7 & Win10 on the same drive but in seperate partitions.
Reason being i would sooner or sadly later have to stop using Win7 anyway so a format and fresh install was impending and dual-booting on the same drive was the easier option.
Fast forward to Sunday 31st May...I was pretty happy with the way things were going, i was 98% happy to make the leap, there were a couple of niggles such as i couldn't get the Win10 taskbar customised to the colour i like but things like that were so minor i was pretty much ready to make the leap...
On Sunday i logged into Win10 and went to Windows Update eager to see what the latest Build had to offer....
After various updates i was upgraded to Win10 build 1909.
What happened next is barely believable, but it happend, and has left me wondering what to do next! Win10 1909 had no LAN/Internet connection at all, i rebooted into Win 7 and there was no LAN/Internet available on that either.
After checking cables and rebooting router/modem etc it was clear that Build 1909 had broken something, and that somehow it had spanned my dual boot, i tried re-installing the network and motherboard chipset drivers within Win10 and Win7 with no joy.
I uninstalled the Win10 Microsoft Updates installed the day before but two did not give the option to uninstall, go figure??
Booted into Win10 and using System Restore, restored to a point before any changes were made, still no joy.
Restored Win7 using System Restore from the day before, no joy again.
I was completely locked out of the internet and local area network despite having dual boot options on two partitions.Using my phone to search the internet for options everything seemed to point at a sytem restore (tried that) or uninstalling updates (tried that but 2 x kb articles could not be uninstalled... not even in Win10 Safe Mode... also begs the question why Safe Mode in Win10 is so hard to find now?).
I gave up at that point and went to bed...
Next morning i woke up and decided that even though i'm not familiar with Win10 i had to do something drastic...Win10 Repair Install with keep Apps and Settings was selected (wording will be incorrect but if you're familiar with the setting you'll know what i mean)...
In reality my foremost thought was how on earth did a Win10 update kill Win7, just let me back into Win7 with working network capability and i'll be happy....
Worst case scenario i have a Macrium backup that i can use to restore Win7 but it's a few months out of date, not the end of the world because my data isn't stored on the SSD.
Win10 Repair Install selected... after the final reboot the Boot Menu has gone!!.... <enter expletives!!>
I can get into Win10 and network functions have been restored including the internet... all my previous custom Win10 setting have been over-written but that's the least of my worries!
I can see that my Win7 partition still exists, i just don't know what state it's in AND i can't access it!
Fortunately i remembered bootrec and was able to rebuild the Boot Menu, boot menu restored.... amazingly after all of the above my Win7 installation appears to be fully functional.
Win7 is now confirmed fully functional after a System Restore now that i can access it again, the only casualty being MBAM, uninstalled and re-installed so even that now works.
Win10 is in a hell of a mess and will stay that way... despite my best efforts i'm still nowhere nearer trusting Win10 which is really sad given i love Windows when it works.