If the error “the requested system device cannot be found” occurs when you try to perform Bootrec operations using a USB installation media, it may not find the correct system partition, which causes bootrec /rebuildbcd device cannot be found Windows 10 issue. You may try this method to set the correct partition.
Step 1. Boot from a UEFI Windows 10 installation (CD/DVD/USB) disk, and in Advanced optionspage, choose Command Prompt.
Step 2. Input diskpart and press Enter to list all partitions and for you to identify EFI partition. Please note that EFI partition is formatted as FAT32 type
Step 3. Input select volume 3 and press Enter (here volume 3 is the volume number of the EFI partition).
Step 4. Input assign letter=m to assign a letter to the EFI partition and press Enter. You may replace m with a letter of your choice.
Step 5. Input exit and press Enter to exit diskpart
Step 6. Then type the following commands in Command Prompt and press Enter after every command.
cd /d m:\EFI\Microsoft\Boot\
bootrec /fixboot
Step 7. Input ren BCD BCD.bak and press Enter to change the name of BDC file from BCD to BCD.bak file.
Step 8. Input bcdboot c:\Windows /l en-us /s m: /f ALL and press Enter to recreate BCD. If the letter you’ve assigned for the EFI partition earlier is not m, then change it in this command accordingly.
Step 9. Input exit to close Command Prompt and restart your computer.
This solution has helped many people resolve the error the requested system device cannot be found Windows 7/8/10 and rebuild BCD, especially for people who use a USB installation media to rebuild MBR.