"Missing Operating System": Parallels might have killed my Windows 10 Bootcamp install?

Thanks, I ran /fixMBR and then ran into trouble with /fixBoot when I got an "access is denied" error. I made a new bootable USB with an older version of W10 and was able to do /fixBoot, but it still booted up to a "missing operating system" message. Some people have said that if /fixMBR and /fixBoot don't work, then I should try to fix the EFI bootloader, but there doesn't seem to be an \EFI\Microsoft\Boot\ folder on the EFI partition.

Hi, Loner T! Thanks for your response, I saw that you have been able to help several other people.

I'm using a MacPro5,1 (2009). I did not change the disk layout/partitioning scheme in any way. All I did was update to Windows 10 from within my previous Windows 7 install, and then later try to open my Boot Camp virtual machine using Parallels (version 10.1.4) which had worked with Windows 7 just before the Windows 10 update. Your assumption is correct, that was my first attempt to boot the W10-upgraded-from-W7 BC installation using Parallels. Now that I look it up, the Parallels website says "Parallels Desktop 10 is a end of life product and it is no longer supported. It is not compatible with the latest Windows 10 build. You can try installing Parallels Desktop 13 trial and create a virtual machine." Stupid of me not to check before using it, but I don't know why trying to run it would have corrupted my W10 install.

Here's my diskutil list. My Mojave install is on "Macintosh SSD", the Windows install is on "BOOTCAMP", and "6 TB WD RED" is being used for storage and backups.

/dev/disk0 (internal, physical):

  #:            TYPE NAME          SIZE    IDENTIFIER

  0:   GUID_partition_scheme            *6.0 TB   disk0

  1:            EFI EFI           209.7 MB  disk0s1

  2:     Apple_CoreStorage 6 TB WD RED       6.0 TB   disk0s2

  3:         Apple_Boot Boot OS X        134.2 MB  disk0s3

/dev/disk1 (internal, physical):

  #:            TYPE NAME          SIZE    IDENTIFIER

  0:   GUID_partition_scheme            *1.0 TB   disk1

  1:            EFI EFI           209.7 MB  disk1s1

  2:    Microsoft Basic Data BOOTCAMP        1000.0 GB disk1s2

/dev/disk2 (internal, virtual):

  #:            TYPE NAME          SIZE    IDENTIFIER

  0:         Apple_HFS 6 TB WD RED      +6.0 TB   disk2

                 Logical Volume on disk0s2

                 D3785C12-CD40-45A3-B2E8-945811803CC2

                 Unencrypted

/dev/disk3 (external, physical):

  #:            TYPE NAME          SIZE    IDENTIFIER

  0:   GUID_partition_scheme            *960.2 GB  disk3

  1:            EFI EFI           209.7 MB  disk3s1

  2:         Apple_APFS Container disk4     960.0 GB  disk3s2

/dev/disk4 (synthesized):

  #:            TYPE NAME          SIZE    IDENTIFIER

  0:   APFS Container Scheme -           +960.0 GB  disk4

                 Physical Store disk3s2

  1:        APFS Volume Macintosh SSD      660.7 GB  disk4s1

  2:        APFS Volume Preboot         22.9 MB  disk4s2

  3:        APFS Volume Recovery        510.1 MB  disk4s3

  4:        APFS Volume VM           20.5 KB  disk4s4

Yes, I've deleted Parallels now. Parallels keeps your license saved in your online account, but it's not really good for much at this point if it doesn't support Windows 10. I can see and access everything on the Windows drive from macOS just fine.

Yes, it shows up as an option, but then it just shows a black screen with "missing operating system" at the top left.