As complete newbie this is current configuration after some testing

Ssd1: Kubuntu 18.04 LTS
Ssd2: Win10
HDD1: NTFS shared disk

Boot was in Legacy mode since this is an old HP Elite 8100 cmt.

Booting sequence is this

- wait a lot more than usual on the Bios screen
- an too fast to seen screen I think Intel something
- wait 10 sec a black screen with just the cursor blinking (no command prompt, only a cursor)
- finally reach GNu Grub 2.02 boot menu. Choosing Ubuntu is OK I can boot Kubuntu. Choosing Windows 10 I enter in
- Windows Boot Menu, where there was 3 Windows 10 boot options (something to do with win partitions?) and a Grub2win option (now deleted I un installed the program)

The first Win10 option was the one I was using for booting Win10 when I was testing Grub2win before installing Kubuntu on my second SSD, but currently enter in the Windows Boot screen with the rotating thing endlessly working, the hdd doing a strange noise and finally telling me an error is there and need to reboot.

I'm able to boot win only in safe mode, but strangely even if is with networking enabled no network is recognized. Also shutting down from Win10 safe mode got an critical error and need to reboot. There is something really odd going on.

So now I have a working Kubuntu installation and completely broken my Win 10, that I need absolutely for my projects.

Any suggestion will be very welcomed.

Apparently your testing of Grub2win wasn't very successful. This is windows software which is installed on the primary windows system partition. I've never heard of it before so know nothing about it. It would probably be a good idea to post more detailed information on your system. The best way to do that is to boot Kubuntu and go to the site below and select the 2nd option to download to Kubuntu using the ppa. From there, read the instructions and download and run boot repair as instructed. Do NOT try to make any repairs but select the option to Create BootInfo Summary and post the link you are given when it finishes here.

https://help.ubuntu.com/community/Boot-Repair

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Thanks for answering and suggestion.

Here the link with the info generated from Boot-Repair

http://paste.ubuntu.com/p/qvdF7nmbzg/

Windows, at least in the past, expects to be installed on the first drive, it can be on another drive as long as it has a boot partition on the first drive. Set it to the first drive, boot the Kubutu live and reinstall grub to the MBR of the Windows drive.

Where did you find Win is not on the "first" drive? I can't really understand that from that long Boot-Repair log.

At the end of the file I discovered this, is that what you mean?

=================== Suggested repair
The default repair of the Boot-Repair utility would reinstall the grub2 of sdc1 into the MBRs of all disks (except live-disks and removable disks without OS).
Additional repair would be performed: unhide-bootmenu-10s fix-windows-boot


=================== Final advice in case of suggested repair
Please do not forget to make your BIOS boot on sdc (ATA INTEL SSDSA2M080) disk!

It's highlighted in red.

BTW:
If it is actually the first drive...

A typical OEM Windows 10 installation should have a WinRE recovery partition, most of which are set up to boot into the recovery partition if it fails to boot a designated number of times in a row, which is usually 3 times. If it does have a WinRE and does not go into recovery mode, and your BIOS does not offer to boot into recovery mode, it is possible your Win 10 recovery settings are disabled and you should be able to enable it from recovery mode.

EDIT: If you do any sort of recovery such as fix boot from recovery mode, it might be best to disconnect the Linux drive first.
SECOND EDIT: Because if it boots into safe mode, means grub is working and booting it.

If the 2.7TB drive is just a data drive and the windows system is on the 256GB drive, you have Grub boot code in the MBR of sda pointing to the Kubuntu boot files on sdc1 (74.5GB drive) and the Kubuntu partition has the correct boot files including grub.cfg with what appears to be a correct menuentry for windows. Do you have a separate boot partition for windows? Looks like it.

Since you are able to select windows from the Grub menu and then it fails, your problem is not with Kubuntu or Grub but with windows. Once you select windows from the Grub menu, you are on the windows partition. In your initial post, you indicate when you select windows you get some error message. You might try booting it again and making a note of the specific error and doing an online search for a solution. If you have a windows installation DVD or a Recovery CD with a Repair option you might try that. If not, you should be able to download something from the microsoft site to repair. Make sure you get a download for the correct version of windows. If you are able to repair the windows and write windows code to the MBR of its drive (sda), you might first install Grub code to the MBR of sdc so you can set it to first boot priority and boot it and update-grub from Kubuntu.

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That was the OP designation, not the systems - the bootinfo shows thusI'm guessing that was a prior Win system that has been upgraded (for want of a better description) to Win10 at some point.
I agree with above - this is a Windows problem, not Linux/grub per-se. One the one occasion I had a broken Win10 update system - caused by a subsequent Windows update - I had to erase Win entirely. No great loss in my case, but it means I have no experience in fixing such a situation.

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I confirm my SSD1, SSD2 etc was just my ID for the various disks, simply reflecting the physical arrangement, I haven't the knowledge in evaluating the logical position.

SSD1 --> Intel 80GB SSD with Kubuntu in the upper slot in the case
SSD2 --> Crucial 275GB with Win10 in the second slot
HDD --> Toshiba 3TB NTFS data disk in the third slot

About current situation some more history can maybe help some expert in suggesting me a path.

Activities on this PC was done in the last two weeks as experiments in preparation for a new PC whose components arrived just today. The new PC will have new components but also cannibalize some from the old one
- New CPU, mobo, ram, case
- new ultra wide screen display
- new SSD for Linux (different from current SSD1 used only for testing)
- new GPU from Nvidia to be used only for compute
- old SSD2, HDD maintaining current Win10 installation and data
- old GPU from AMD to be used to drive the display
Being dedicated to data science prototyping (not development, the goal is just testing and understanding new tools to promote new projects) he will have Linux boot on one (new) disk and Win on the old one to maintain some projects.

These the activities performed on current old PC

1. The original PC configuration was SSD2 + HDD with only Win10, perfectly working
2. Changed GPU from AMD to Nvidia, cleaning drivers before using Display Driver Uninstaller. Win10 perfectly working after reboot.
3. Tried booting from Linux live USB, failed to do that I tried to install Grub2Win to try booting from a Linux iso file I loaded on SSD2. I was never able to boot from this, even if Grub2Win attempted to do it and in fact generated a swap and other files on SSD2. There was a lot of forced shutdown in this situation when I remained stuck, in theory no OS was anyway loaded. Win10 installation tested and still correctly working.
4. discovered I can actually boot from USB (my fault creating the USB disk using Rufus), so started testing various linux distro using live USB so no change on PC. Win10 working. Didn't removed Grub2Win nor the Linux iso and related files from SSD2 (maybe a fault)
5. Added SSD1 to the system (an old SSD I was not using) and installed there Kubuntu. From there I tested only using Kubuntu and never entered again on Win10.
6. Arrived and installed new ultra wide screen display (previous testing on FullHD TV), working on Kubuntu. Noticed the startup time was getting longer with the multiple steps I was telling at the beginning

And here we are.
Last update last boot actually Win10 started for the first time in a regular way, just after a lot of time working on the Win boot screen. I didn't changed anything in the configuration to achieve that. Updated some display drivers, reboot and now again Win10 didn't start, with error message INACESSIBLE BOOT DEVICE.
Also tried removing SSD1 with Kubuntu installation and the system didn't boot at all and reach only a grub screen with no option and a command line.

I suppose something is wrong with boot config, since clearly Win installation is still working.

I'm not sure what you are referring to here. You keep referring to SSD1, SSD2 and HDD yet you have the output of boot repair and clearly know the size of the drives.

sda is the 256GB drive with only windows
sdb is the 2.7TB drive with only windows
sdc is the 74,5GB drive with only one Linux partition

Yes, that is correct and expected behavior since as I pointed out in my previous post, you have Grub code in the MBR of sda pointing to Kubuntu on sdc1 and since almost all the necessary Grub files needed to boot on the system partition (the one you dis-connected) it will not boot.

You're not clear about the display drivers you updated but it seems from your post that this was on windows. If that's the case, there's nothing your Kubunt or Grub can do to fix that.

I'd repeat my suggestion to install Grub from Kubuntu to the MBR of its drive which is shown in your output as sdc.
After that, use the methods suggested to repair your messed up windows install. Install windows code to the MBR of the windows drive.

Your last post is confusing as you first state that "Updated some display drivers, reboot and now again Win10 didn't start" and two sentences later you state "since clearly Win installation is still working". So is it working or not??

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Apologize for my ignorance, I simply didn't have the knowledge to name disks with their right name. Now understand what you mean.

Will try to install Grub on MBR of sbc (Kubuntu drive). I suppose I need to delete it from sda?

Yes display drivers was installed on Windows. I'm just saying Windows installation seems not completely brocken, since for a time started completely normally, without doing any change in the config. Can't understand why after that returned on the same wrong behavior.

Also considering the fact I just received the new PC components I want first of all to recover a working Win10 installation since will need anyway to reinstall Linux on the new ssd.

Using Boot-Repair I installed grub also on usbc, also tried to use Win10 boot options to repair without success. Current configuration is visible here

Now reading about the different Boot-Repair options in "Other options" tab I discovered the option "Repair Windows boot files" that is disabled.

There is any way to make it work, and make sense to use it?

Your windows system is messed up for whatever reason, the Grub2Win thing or something else. You can boot Kubuntu and there is no problem with the menuentry in grub.cfg for windows so you need to repair the windows booting. You should install Grub to the MBR of sdc where you have Kubuntu as a safety precaution in the event your attempt to repair whatever the problem is with windows fails. You don't need to delete anything from sda, you need to use some windows software to repair the bootloader and put windows code in the MBR A windows installation DVD, recovery CD or similar software which you can download from microsoft or other sites.


??Does that mean you can now boot windows?? If so, what's the problem?

What is usbc?? What win10 boot options to repair? Do you have the necessary software for windows? Were you trying to do this with boot repair? Boot repair is designed primarily to repair Linux system boot problems and although it might be successful in making minor repairs to a windows system, using windows software to repair windows is a better solution. If your windows filesystem is corrupted or boot files are missing/corrupted, boot repair won't help nor is it intended to.

Your most recent boot repair shows that you have now installed Grub code to the MBR of sdc so selecting that drive as first boot should give you the menu which should show 2 windows options. What happens now when you select either?

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Windows installation worked just one single time after installing Linux, no idea why, was random after many start ending in BSOD. After that always ended in BSOD again.

Of the two options I have now for booting Windows both bring me to BSOD, the first one after asking for repair, the second directly .

New PC is 90% built, starting thinking to reinstall Windows, even if will loose a lot of time and just think a better skilled than me will be able to make it work, I think there is just no reason why started doing this. But so it is.