Windows 10 Upgrade caused inaccessible boot device

 Inserted Windows 10 USB

Used command prompt from this article

• Bootrec /fixmbr
• Bootrec /fixboot
• Bootrec /rebuildbcd

 However the last command returns:

The requested system device cannot be found.

See if a disk is MBR or GPT

First. Determine wheteher you have GPT disks or MBR disks.

GPT work on the newer UEFI mode boot in BIOS, whereas MBR disks use legacy mode.

diskpart

list disk

See if it has a * in the GPT column

https://support.novastor.com/hc/en-us/articles/360011403653-How-to-repair-the-EFI-bootloader-on-a-GPT-HDD-for-Windows-7-8-8-1-and-10

These commands succeeded.

However on booting it now complained:

Your PC/Device needs to be repaired

File: \BCD

Error code: 0xc0000098

In the BIOS turned disabled CSM Support (this supports legacy [MBR] boots).

As I had GPT disks I had to be sure I was only trying to load in UEFI mode.

Restored the EFI bootloader using this
https://support.novastor.com/hc/en-us/articles/360011403653-How-to-repair-the-EFI-bootloader-on-a-GPT-HDD-for-Windows-7-8-8-1-and-10