So, I installed Windows 10. I didn't liked it (at all) so I installed after that Windows 7 (on a different partition).
Now after 6 months I would like to see if Window 10 has improoved a bit, but Windows 7 won't show it in its boot menu. So, I cannot boot into Win 10 anymore.
Solution: bcdedit
Run the command prompt as administrator. Type bcdedit and it will show all installed operating systems (that it can recognize).
In my case (Win 7 primary, Win 10 'lost' partition) the output is this:
C:\>bcdedit
Windows Boot Manager
--------------------
identifier {bootmgr}
device partition=K:
description Windows 10
locale en-US
inherit {globalsettings}
default {current}
resumeobject {24334bef-7a25-11e5-866b-bca012dbdc37}
displayorder {current}
toolsdisplayorder {memdiag}
timeout 0
Windows Boot Loader
-------------------
identifier {current}
device partition=C:
path \Windows\system32\winload.exe
description Windows 7
locale en-US
inherit {bootloadersettings}
recoverysequence {24334bf1-7a25-11e5-866b-bca012dbdc37}
recoveryenabled Yes
osdevice partition=C:
systemroot \Windows
resumeobject {24334bef-7a25-11e5-866b-bca012dbdc37}
nx OptIn
As you can see the current OS is Windows 7 on C: partition.
First of all, make a backup copy of your boot entries, in case you fuck up something:
C:\>bcdedit /export c:\bdc_entry_backup
The operation completed successfully.
Then type the following command. It will add Win 10 (identified as 'bootmgr') at the list of available bootable OS's:
C:\>bcdedit /displayorder {current} {bootmgr}
Windows Boot Manager
--------------------
identifier {bootmgr}
device partition=K:
description Windows 10
locale en-US
inherit {globalsettings}
default {current}
resumeobject {24334bef-7a25-11e5-866b-bca012dbdc37}
displayorder {current}
{bootmgr}
toolsdisplayorder {memdiag}
timeout 0
Windows Boot Loader
-------------------
identifier {current}
device partition=C:
path \Windows\system32\winload.exe
description Windows 7
locale en-US
inherit {bootloadersettings}
recoverysequence {24334bf1-7a25-11e5-866b-bca012dbdc37}
recoveryenabled Yes
osdevice partition=C:
systemroot \Windows
resumeobject {24334bef-7a25-11e5-866b-bca012dbdc37}
nx OptIn
Done.
_________
More details about bcdedit here: https://msdn.microsoft.com/en-us/library/windows/hardware/ff541231%28v=vs.85%29.aspx#editing_the_boot_menu_in_windows_vista_and_later
