Multiple Win 8.1 x64 Pro install on Boot Camp Assistant 6.0

Thanks Loner T, let me give it a try and I'll feed you back with outcome.

I am back and with me the solution to my problem... Before I proceed to writing anything, read below to know more about this minor detail that could save you hours of brainstorming and headache.

After wiping OS X El Capitan 10.11.1 from my MBP, I tried installing OS X Yosemite 10.10 but it I was unsuccessful😮😠 after several uncountable tries. I also tried different OS X Yosemite 10.10 dmg installation files but I was still unsuccessful. None of them will boot; it will simply display a barred symbol on the screen; like the one right here:

Considering that I have a really silly😝 internet bandwidth, it became really annoying to have to spend days downloading and unsuccessfully trying to install Yosemite. I felt my only option is to use the Internet Recovery, of which am not really fond and avoid; sine am only seeking for non-internet based solutions. I went back reading on the internet and the was some controversy on Internet Recovery: some said it restores original OS which came on your Mac others said it loads more recent OS unto your Mac. At this point even more confused 😕. So it came to my mind, a simple fundamental principle which Apple applies on iPhones... you cannot downgrade an iOS version of an iPhone to a version prior to that which was released with the iPhone itself, even if the iOS version is an intermediary (minor) release. After this moment, I verified and saw all previous OS X Yosemite I had downloaded were actually 10.10 NOT 10.10.5 which is what came with my MBP Retina mid 2015. Finally I gave a last try, searched around and got a dmg file for Yosemite 10.10.5 and after creating installation USB... got my MBP running back on OS X 🙂😁😎➕✅

Once i got Yosemite 10.10.5 running, I was able to install Windows 8.1 Pro x64 on my MBP using older version of BCA which comes on Yosemite 10.10.5. Since I already had my BC drivers and the Windows installation USB created with BC, I plugged-in both USB keys, went in straight into installation using last option in BCA. After installing Win 8.1 I simply installed my drivers by running setup file found in USB key with BC drivers. Now, I've got both OS X and Windows OS up!!

Finally, in order to restore my MBP and all its data and content, I upgraded back to El Capitan (since you cannot restore a backup of El Capitan on Yosemite) using a dmg file which I had previously downloaded, I copied it unto my desktop and simply executed it. Then, I followed the on-screen instructions and got back unto EL Capitan😍. From there, I pulled up an external 2 TB hard drive I used as backup with Time Machine and executed Migration Assistant to transfer back all of my files. It successfully restored my MBP and looks like nothing ever happened to it (like new), plus I have Windows OS up in BC partition. To conclude, I then installed the update to upgrade again from El Capitan 10.11 to El Capitan 10.11.1.

As far as I am concerned, what I can say is:

1. If you are downgrading your Mac OS version, please make sure you get a Mac OS dmg file version that was originally shipped with your Mac, this may save you a lot of stress and headache.

2. Never neglect the minor releases version counts attached the main OS version release; e.g. in Mac OS X 10.x.x the x.x are very important when searching, downloading and performing a downgrade install back to factory settings on your Mac.

3. Different mades of Mac computers support different least Windows OS versions. Mine will only support as from Win 8 x64 upwards, while other Macs will support Win 7 x64 and so on... Please pay close attention to what BCA displays.

4. I might be too explicit in my explanations but I believe this information is of good help and provides precious details needed by all novice just like myself, thanks.

P.S. Ok, to download all these versions of OS X, I installed Windows 8.1 Pro to power up the MBP, I used my BC drivers to install my drivers and it worked fine. I was then able to connect via WiFi and browse over the internet to search for solutions. For some few days I had an entire Windows-Mac, hahaha.😁😁😉

Loner T, it didn't really workout simple in two steps as you wrote... many more intermediary steps were required and involved. Still though I will appreciate if more details can be provided as often as needed for newbies like me, thanks.

Loner T you should be awarded the ApplePedia trophy🙂😉, you've got amazing solutions and also the reliable links to internet material, thumbs up!! Just FYI, I have never updated the firmware on my Mac, so the original build still works.

I agree with you, EL Capitan is just too way below expectation and really needs a lot of amendments prior to officially rolling it out on critical Macs. Yes, I have gotten back on a working Mac and as you rightly say, I frequently and preciously keep good backups, as no one knows what this El Capitan is capable of yet, talking in terms of disaster, 😁😁 hahaha.

Sure, I will post back if I encounter further issues, again thanks for all.