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It's important to know if your original Windows installation booted in BIOS/CSM/legacy mode or in EFI mode. Most Windows 10 installations on Macs boot in EFI mode, but that's not universally true. If it booted in BIOS mode, then the disk must have once used a hybrid MBR, and your macOS upgrade probably either replaced the hybrid MBR with a protective MBR or it wiped out the Windows BIOS-mode boot loader. If Windows booted in EFI mode, then maybe the Windows EFI-mode boot loader was damaged or replaced, or perhaps something about the Apple boot menu is messed up. The repair method will be entirely different depending on which boot mode was used, and trying to repair in the wrong way could just make matters worse. If you don't know the original Windows boot mode, I suggest you examine your to see if it holds any Windows boot loader files. (They'd be in the EFI/Microsoft directory tree, if they exist at all.) If there are Windows boot files on the ESP, chances are it had been booting in EFI mode.

If not, then either it was booting in BIOS mode or the macOS upgrade wiped those files out. Thus, lack of those files is less diagnostic than their presence. Even if Windows had been booting in BIOS mode, you might be able to convert it to boot in EFI mode by installing an EFI-mode boot loader. Computer repair near me for mac. This is described here, among other places: Note, however, that this blog post describes a full conversion on UEFI-based PCs. Some of the work (like converting from MBR to GPT) is already done on a Mac (although you may need to convert a hybrid MBR to a protective MBR).