How to upgrade 1511 from Home to Pro?

You might try disconnecting one drive (0), doing what you want with the other drive (1), then do the same with the one drive (0). Use BIOS/UEFI to manage the boot order. I had similar issues with triple boot, 7, 8.1 and 10 all pro versions.

Good luck.

Use my Pro key if you want. Works every time. But my PC got 8.1 when bought with factory settings. I did the free upgrade to Windows 10 home. Then i changed product key and it will auto install Windows 10 Pro. If you want i will help you? Just sent me an email: ******@gmail.com. one4allall4one

Oddly enough I've just been presented with an unexpected opportunity to repeat this entire scenario on a computer which is nearly a carbon-copy of the one I just did (and experienced all the problems with).  It is a Dell that originally had Windows 7 Home Edition on it which was later upgraded to Pro via a Win 7 Pro Pack and then on up to Win 10 Professional.   I was able to go back and find the original MCT for the first version 1511 update (the one MS later pulled due to some minor problems which in reality didn't amount to a hill of beans) and used it to create a flash drive upgrade media.  Sure enough, at the install screen it gave me the choice of either Home or Professional Edition.  I chose Professional of course and after that everything went as smooth as silk.  I was asked for a product key 1 time up front (which I skipped over) and by the time the Windows 10 Desktop appeared it had already been fully activated; in other words it worked flawlessly and the whole process went off without a hitch.  If there's any way you can communicate this experience to Microsoft I wish you would, because this is the way it should work each & every time under this particular scenario, that being one in which a computer has been previously upgraded to Pro Edition when it had an earlier qualifying operating system installed on it and then subsequently upgraded to Win 10 Pro (which had to be wiped out, for whatever reason, and a clean re-install performed).  Given the choice of installing the correct version of the OS from the get-go made all the difference in the world; it enabled the OS to ignore the embedded OEM product key and in-turn allowed the Microsoft Activation Servers to immediately verify that the correct OS version had indeed been previously installed and properly activated.  In other words, given the proper sequence of events the system worked exactly as Microsoft seemingly intended it to, before they ill-advisedly monkeyed around with it and messed everything up.  As I said earlier, "If it ain't (that) broke, don't fix it".  Thanks for all your help in at least trying to get MS to do the right thing.  Please let us know their commentary, if any.

Thanks, Travis.  I've passed this along.

I appreciate your response. I must not have been entirely clear though; Andre did indeed provide me with the product key you mentioned and as soon as I entered it Windows (curiously) went through the same Home-to-Pro upgrade process, just as if I'd utilized a Pro Pack. After all the usual gyrations and various reboots when it finally came back up it was back to Windows 10 Pro and it immediately activated (re-activated, actually) when it connected with the Internet.  But what a nightmare to be put through, not to mention how is one supposed to know all these arcane machinations that you have to go through to get back to where you had already been before?  As I think I mentioned, I've done several other clean installs on previously-upgraded Dells identical to this one and have never encountered this particular problem; as far as I recall Win 10 Pro was installed without a hitch and immediately re-activated itself.

However, it's obvious that many people will fall victim to this same scenario and many, if not most, will have no idea how to visit these Forums, much-less how to solve the problem.  I hope you can use my (and the many others you've encountered) case studies to convince Microsoft that "it wasn't broken so don't fix it", unless they can come up with another solution to the OEM embedded product keys.  And in Dell's particular case they don't even offer Win 10 Pro on a lot of their machines and the ones they do are only available in 64-bit versions; in my case I have a unique requirement for 32-bit Windows which even adds a greater challenge to it since it's near-impossible to "downgrade" from 64 to 32 bits.  Please keep hammering them to either go back to the way it was or build-in some better sensing technology to allow a person to override the OS's initial guess as to what version should be installed.  Or make it where the installer will check-in with the Windows Store (database) to verify what the computer's most recent installed version of the OS was and then re-install that particular version, overriding whatever may be hard-coded into the BIOS.  I can guarantee that this is will be a continuing problem that isn't going away anytime soon.  Please keep us posted out here as to what the Brain Trust in Redmond is up to as regards this issue, okay?