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I've been trying to set up my ThinkPad T420 to boot Windows 10 Technical Preview on disk 0 (/dev/sda) and Ubuntu on disk 1 (/dev/sdb). I've reinstalled many times, and now I'm having trouble booting from disk 0. Booting is very slow, and hangs unless I bring up the boot manager and select disk 0. The Boot Manager entry used to boot Windows, but doesn't anymore. I also can't boot from my USB drive anymore.

I believe this is because the boot information is a little screwed up. In one of my many reinstalls, a Ubuntu link somehow was added to Windows Boot Manager - I think by boot-repair, though I'm not sure. Despite deleting all partitions in DiskPart, running Clean, and recreating everything, that Ubuntu entry is still there. So I think the data is corrupted, but I can't figure out how to clear it.

Why is the boot manager data not cleared when I delete every partition on the disk and run "clean"? Where is this information stored? Is EasyBCD the only way to access and edit this data?

permutations
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  • Again as in previous question. UEFI or BIOS. With BIOS normally you do not need to clear a boot loader as you just install a new one. But with UEFI it is different. http://askubuntu.com/questions/63610/how-do-i-remove-ubuntu-in-the-bios-boot-menu And Boot-Repair nor any Linux edits Windows BCD. But Windows does link BCD to UEFI menu. – oldfred Apr 24 '15 at 03:40
  • I don't follow what you're saying. Both my hard disks use UEFI partitioning. The link you gave says to install something under Ubuntu to edit the Windows BCD. I don't understand that. This can be edited directly in Windows using bcdedit (comes with Windows). In any case, the mysterious part is that boot configuration data is stored in a hidden partition in Windows, so if all the partitions are deleted, how is this information retained? – permutations Apr 27 '15 at 20:31

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I fixed the BCD problem with BCDEdit - the command line version that comes with Windows. But I still can't figure out why deleting all the partitions and running the "clean" command doesn't delete the database. I read that the database is stored in the hidden system partition. So if the partition is blown away, how is the data retained?? Mysterious.

I have a working dual boot now - two disks, both using UEFI and GPT partitioning. But I have Setup configured to allow both UEFI and BIOS because otherwise I can't boot from USB drives.

I added a Ubuntu entry to the Windows boot manager. That chains to Grub2 which loads Ubuntu. But it's more efficient to do it the other way - boot into Grub2 and select Windows if I want it. Originally that went to a second menu, too - the Windows boot selection menu - containing just one item: Windows 10 Technical Preview. I got it to bypass the second menu and boot straight into Windows with this command, issued from the command line as an administrator:

bcdedit /set {bootmgr} displaybootmenu no

Everything is working now.

permutations
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