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I have Windows 10 on an SSD in a DVD caddy (/dev/sdb) and Ubuntu mate on an HDD (/dev/sda). Both are Legacy BIOS/MBR on a Dell Laptop.

I made a dual boot set up and tested putting GRUB on the Linux disk /dev/sda and again tried with GRUB on /dev/sda1. I used EasyBCD to add the location of GRUB to the Windows Bootmgr.

With this set up when I turn on my laptop the Windows 10 boot menu gives me the choice of Windows 10 and Ubuntu.... IF I choose Ubuntu my laptop first does a quick reboot then I get a GRUB2 menu where I can choose Ubuntu or Windows.

Can Windows Bootmgr just load Ubuntu without needing a second reboot? Is there anyway to make this happen? Other methods or boot managers?

What I have tried
I tried placing the Ubuntu bootloader code in /dev/sda and also in /dev/sda1. I was thinking that putting the GRUB2 stuff in a partition might allow Windows to load the Linux OS without needing an extra boot after I select Ubuntu from the Windows bootmgr menu.

What I read elsewhere
GRuB can load Windows without rebooting. The grub bootloader can only be loaded by the system BIOS when the computer boots.

It seems that dual booting Windows and Linux involves "Chain Loading". But does chain loading always require extra reboots?

GRUB2 can load Windows without needing an extra reboot. Can Windows Bootmgr load Ubuntu without needing a second reboot?

Cheers.

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Windows Bootloader does not and can not load any Linux install, and neither has any version of GRUB loaded Windows. They just transfer control to the initial sector of the bootloader for the other OS. An additional boot menu may or may not be shown, depending on the existing boot options. Having a second boot menu shown does not constitute "rebooting". If you don't want the GRUB menu to show when transferring to it from Windows, all you have to do is change the GRUB options.

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  • Cheers. Yes I know I could skip the GRuB menu. I'm sure I'm getting some terminology wrong. But my question remains... Dual booting Windows 10 and a Linux OS such as Ubuntu with Windows Bootmgr vs GRuB. Have you ever done that? Do you notice that the machine does NOT restart when you select Windows from the GRuB menu but it DOES restart if you select Ubuntu from the Bootmgr menu? That's what I observe on my laptop. I hear the fan stop for a moment and the Windows startup animation appears. Is there anyway to stop that second boot when selecting Ubuntu from Windows Bootmgr menu?
    – FlexMcMurphy
    Apr 15, 2021 at 8:57
  • Actually the behavior is not limited to booting to grub/linux from WBM. I'm experiencing it when I try to do multi-boot for two Windows installations (one Home one Enterprise though). It seems that WBM only display its menu after a certain boot procedures has done for its default osloader entry. So booting is done very quickly for the default entry from the point you choose the item, while booting a non-default entry will require rebooting.
    – Tom Yan
    Apr 20, 2021 at 14:15
  • @FlexMcMurphy I'm not sure if there's a way to avoid the behavior. Most likely it's defected by design. Perhaps you should just use another bootloader / boot manager to load Windows Boot Manager instead of the other way round. You can try setting Ubuntu as the default osloader entry with bcdedit and see if it helps though.
    – Tom Yan
    Apr 20, 2021 at 14:22

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