How to reinstall OS X on Mac Mini running Windows 10

I have 8 Mac Mini's all running Windows 10 as a piece of software I required to run only supported Windows. The IT assistant who managed the installation of Windows regretfully did not use Bootcamp or similar software so the computers operate just as Windows machines and use a standard Windows keyboard. I now want to get them all back to Sierra OS to I can use Bootcamp properly but despite creating an installer USB drive using my own Macbook Pro, I cannot get the computers to boot to the drive and therefore reinstall OS X over Windows. I have tried using Advanced Startup options through settings and selecting the USB drive but Windows still boots normally. All support threads I can find online relate to either installing OS X on a PC or restoring OS X on a Mac, cannot find any advice on reinstalling OS X on a Mac running Windows 10. Any support would be gratefully appreciated.
 

jobeard

Posts: 13,863   +1,756
Booting from an external device:

Step 1: Turn on your Mac by pressing the power button by choosing Restart in the Apple menu, or restart it if it's already on. Step 2: Press and hold the Option (⌥) key immediately upon hearing the startup chime. Release the key after Startup Manager appears.
 
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Thanks jobeard, turns out I was making the rookie mistake of using a Bluetooth keyboard. Swapped it for a wired one, held down alt on restart and found out boot camp was installed after all and was able to reboot in OS X. Then downloaded the boot camp restoration files from boot camp assistant, saved onto USB and rebooted back to Windows, installed the files and all working well now. Thank you for taking the time to reply!
 

jobeard

Posts: 13,863   +1,756
Thanks jobeard, turns out I was making the rookie mistake of using a Bluetooth keyboard.
That's quite common -- until you realize that BT comes on way-late in the boot sequence and you need to control the Boot. Bet you don't make that one anytime soon...
 

condie

Posts: 8   +0
I got a similar problem I made my Mac Mini a Windows only machine,, Now I cant restore it as a Mac with Bootcamp. None of the startup keys combos works,
 

jobeard

Posts: 13,863   +1,756
The issue is not which key, but that the signals don't get there until WELL AFTER BOOT.
The BIOS is not seeing the keys early enough
 

condie

Posts: 8   +0
I see USB stuff on eBay but don't know if they'll format and install on this Mac which thinks it's a PC
 

jobeard

Posts: 13,863   +1,756
By carefully connecting the Mini and then selecting the right device -- it can be confusing :-(
 

condie

Posts: 8   +0
You jest!!!

AppleSupport just emailed instrctionS on creating an install USB. I'll keep y'all posted.
 

jobeard

Posts: 13,863   +1,756
In reply to
How do I format a Mac partition?
I said
In Mac OS X, how do I partition my hard drive?
Warning: In versions of Mac OS X prior to 10.5, or when changing the partition scheme in Mac OS X 10.5 and later,
partitioning your hard drive with Disk Util...kb.iu.edu kb.iu.edu
and cautioned
By carefully connecting the Mini and then selecting the right device -- it can be confusing :-(
you doubted that reply
You jest!!!

AppleSupport just emailed instrctionS on creating an install USB. I'll keep y'all posted.
yet, in the referenced document, it says
If your disk doesn't appear in Disk Utility

If Disk Utility doesn't show a sidebar, choose View > Show Sidebar from the menu bar.

If Disk Utility shows the sidebar, but your disk doesn't appear within it, disconnect all nonessential devices from your Mac. If the disk is external, leave it connected, but make sure that it's turned on and connected directly to your Mac using a good cable.
Hopefully, all is going well with the reinstall