I booted a Ubuntu 16.04 Live USB and installed onto an external HDD. In the partitioning step I had it install grub to the external HDD, not my internal Windows 10 HDD. But when I rebooted the computer, it boots to a grub command line.
In side the UEFI, there is still a Window Boot Manger option, plus 2 Ubuntu boot options. I can still boot to Win10 through the WBM, but the 2 Ubuntu options do nothing.
How can I remove all traces of Grub and the 2 Ubuntu boot options from the Win10 HDD?
It sounds like GRUB is broken.
Try booting into the live CD again, open a terminal and type sudo update-grub.
This should fix GRUB. Then the Ubuntu boot option in UEFI should go to GRUB which should let you choose between Ubuntu and Windows.
I don't think that removing all traces of GRUB from your hard drive will solve your problems, unless you don't want Ubuntu anymore. WBM does not support Ubuntu but GRUB does support Windows, so it is preferable.
Ok I followed https://askubuntu.com/a/240831/440238 and deleted the Ubuntu boot stuff from Win10's efi partition. The Ubuntu uefi entries are now gone and Windows boots like normal
I've had problems with Ubuntu before where it installs boot files to other HDDs when you specifically tell the installer not to, it really should be fixed.
