Windows installed on HD A. I installed deepin on HD B (with the bootloader being installed to HD A). The boot menu only showed OSes installed on HD B, even when pressing "reset" in the boot menu control center. I had to manually run "sudo update-grub" for it to detect the OSes on HD A.

I'll also tag on a feature request: You should be able to drag and drop the listed OSes in the boot menu settings to change their display order. Even though you can set the default boot OS, it's nice to have Windows listed as the first entry on a family computer where non-tech savy people use it. I usually manually edit grub.cfg to make this change.