Ubuntu – Dual-boot Windows 7 and Ubuntu on two SSDs with UEFI

Selectively unplugging one disk or another is a trick that some people used in the BIOS era, but EFI's design makes this approach less useful. This is particularly true for some computers, which may erase boot manager entries when they're found to no longer point to valid boot loaders, which of course will be true when you unplug a disk to which they point. Thus, I strongly recommend against using this procedure when using an EFI-based computer.

The last I heard, EasyBCD was useless, or nearly so, on EFI-based computers. (That said, I haven't looked into it recently, so it could have changed.) Note that EasyBCD is a third-party Windows tool for managing the Windows boot loader and boot manager, which is known as the BCD. The BCD itself can be used to chainload to a Linux boot loader; however, I know very little about how to configure it to do so.

Most Linux users run GRUB 2 as their boot manager under EFI; however, other options are available, as outlined in this question and its answers. Ideally, the simplest procedure to get GRUB 2 working is:

1. Install Windows
2. Install Ubuntu

The Ubuntu installer should detect Windows and add it as a boot option to the GRUB menu. Juggling disks, skipping the GRUB installation in favor of something else, and other advanced trickery is most likely to cause new problems. That said, there are cases when you might want to take an alternative path, such as if you dislike GRUB and want to use something else. In such a case, though, you should research the issue before beginning.

Also, note that neither Windows nor Ubuntu should delete the other's boot loader. What both will do, however, is to place their own boot loader (BCD or GRUB) as the first one in the EFI's boot manager list. You can modify this ordering with EasyUEFI or bcdedit in Windows, with efibootmgr in Ubuntu, or with some (but not all) computers' built-in firmware configuration utilities. If you don't understand how all these pieces (the firmware's boot manager, BCD, GRUB, and perhaps other tools) fit together, you're likely to become confused. To that end, here's some suggested reading:

• Adam Williamson's blog entry on how EFI works -- This page describes the theory behind EFI-mode booting.
• My page on installing Linux on EFI systems -- This page is a more practical introduction to EFI-mode booting than Adam Williamson's blog.
• My page on EFI boot loaders for Linux -- This page describes EFI boot loader choices for Linux systems.
• The Ubuntu community wiki entry on UEFI -- This page is a bit disjointed, but is the most Ubuntu-specific introduction to EFI and its issues.

For specific advice on how to proceed, if you can boot Ubuntu right now, try doing so (with the Windows drive also connected) and then type sudo update-grub at the command line. That will re-run the GRUB configuration scripts, which should cause it to detect Windows and add it to the GRUB menu. If that doesn't work, then you need to be more specific about what's not working -- for instance, is Windows not appearing in the GRUB menu, is it appearing but failing, and if it's failing, what error messages (if any) do you see?

Change the SATA settings in the UEFI setup utility from RAID mode to AHCI mode, and use UEFI for both Windows 10 and Ubuntu 16.04 which are installed alongside each other as a dual boot, but in Windows you have to first install AHCI drivers. Windows AHCI instructions: https://askubuntu.com/a/963100/ & https://askubuntu.com/a/1148150/.