I have a hard drive with Windows 10 on it and a spare one I installed Ubuntu on. I wanted to have it automatically boot the hard drive with Windows 10 on it instead of showing a grub menu and having to select what OS to boot into and then if I wanted to use the Ubuntu drive, I could just hit Esc key when my computer boots and select the Ubuntu drive and then have the grub menu pop up. Now when I select my Windows hard drive to boot it, it brings up grub rescue prompt but if I select the Ubuntu drive it brings up the grub menu to select the OS. I unplugged the Windows 10 hard drive when I installed Ubuntu so why is it being affected by the Ubuntu install when I try to boot that hard drive?
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I just set this up for my Dell laptop. I installed Ubuntu 14.04 LTS (not live) on a USB flash drive. When I boot my laptop, it boots to Windows 10 by default. Then, when I plug in the U-14 flash drive, I press F12 for the boot menu and choose Ubuntu.
In the computer BIOS, I turned OFF secure boot. I set Boot Options to Ubuntu, Windows Boot Manager, none and none.
Then I plugged in the Live and target USBs and booted to the Live USB. I used Gparted to partition a 2GB swap partition at the end of the target drive and formated the rest of the drive as EXT4, /. After applying the changes, I changed the flag of the main partition to boot.
When I chose the partitions to install I chose to install U14 to the / partition. I also changed the Boot option to SDB (not a partition.)
When I was done, I booted to the BIOS and turned ON the security boot.
When I boot, I press the F12 key and get a choice to boot to Windows or Ubuntu. If I remove the USB, it default boots to Windows.
I can also change the BIOS boot order to Ubuntu and then Windows. Once set, I can boot to Ubuntu as long as the flash drive is plugged in before I turn on the computer. But if I boot without it, I will have to use the boot menu to boot to Ubuntu until I change the BIOS default order again.
Ubuntu always boots through the Grub menu. Windows does not use the Grub menu.
I hope this helps.
