~30% – but when it works, you're done in 5 minutes.
diskpart list volume exit Look for the volume labeled or System Reserved . Often the OS drive is D: in recovery mode (because the recovery environment uses C: for itself). Note the drive letter (e.g., D:). Step 4.3 – Rebuild Boot Configuration Data (BCD) This fixes 80% of boot errors: windows server 2012 r2 boot repair
From Command Prompt:
bootsect /nt60 sys /mbr Then retry /fixboot . If /rebuildbcd doesn't find any OS, rename the old BCD and create a new one: ~30% – but when it works, you're done in 5 minutes