Windows – Can’t install Windows 7 on extended partition

Delete the partition, then skip format, just choose the unformatted partition as the target, let Windows 7 setup do the rest.

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I got this same error, setting the hard disk as 1st (and even removing the USB stick from the boot order) did not help. As far as I can tell, this is some interaction in the BIOS that prevents the win10 installer from thinking that the installation target drive is bootable, perhaps due to the particular USB key I was using (a USB 3.0 Kingston DataTraveler). I solved it by creating an "install partition" on the target disk using diskpart from the command line (shift+F10 in the installer):

> diskpart
diskpart> list disk
diskpart> select disk 0   # Choose the disk you want to install to
diskpart> clean           # will delete everything, hope you have backups!
diskpart> create partition primary size=100000 # size in MB
diskpart> create partition primary size=5000 # temporary install partition
diskpart> format fs=ntfs quick
diskpart> assign          # assigns a drive letter, it was D: for me
diskpart> active          # makes the install partition bootable
diskpart> exit

then copy the installer files from the USB key to the temporary partition:

c:\> xcopy c: d: /e /h    # /e for recursive with empty, /h for system files

and reboot, remove the USB key, and boot from the temporary install partition. The win10 installer will detect that the disk is bootable this time through & will let you install. When you're done with the installer you can remove the temporary partition and resize your main partition to take up the whole disk.