Trouble Free Dual Booting in the SFF or MT Dell OptiMac
As has been stated in the
Multi-Booting forums numerous times, it's much better to dual or triple boot on separate drives to avoid many serious complications. The worst combo is Windows and mac OS installed on the same drive. It can work but doing this has the highest probability of data loss and corruption of the bootloader and macOS itself. Using the same HDD or SSD for each OS makes updates and backups/clones a rather complicated matter. Because of the APFS file system you should never install macOS Mojave, Catalina or Big Sur on an HDD anyway. It boots and runs so slow that it makes macOS almost unusable.
If your are an SFF owner or plan to get one, you can easily dual boot with Windows (GPT/UEFI) on the 3.5" HDD and macOS installed to a 2.5" SSD. If you do want to
keep the Windows HDD connected internally, full time, here's what I recommend that you do. Leave the Windows HDD connected when you're booting into macOS. It shouldn't create any problems that way. When you boot into Windows though, with your SSD also connected, that sets you up for problems. When you want to boot into Windows, disconnect the
Sata data cable to your macOS SSD. That way Windows will never overwrite the EFI partition with your bootloader on it. No Sata data cable connected to the SSD means Windows cannot mess with that most important EFI partition. Run your Windows updates with the SSD disconnected also. It just makes multi-booting Windows and macOS much more trouble proof.
Of course
if you only boot into Windows a few times per year it makes no sense to leave it connected to Sata data and power full time. It just wastes energy. Disconnect the drive and only connect up when you need it. Again, the macOS SSD should be disconnected when running Windows or your OC bootloader may get overwritten.
Here's how easy it is to slide in a 7mm thick 2.5" SSD above the mechanical 3.5" HDD and below the CD/DVD drive. You should already have the extra Sata cable to use and hopefully the other full size Sata power connector.