I would like to know if anyone can help me with this. I would like to add Windows to my build on a separate drive. I am running El Capitan. Can I do this? I have been searching on the forums but it seems like every forum I found talks about fresh install on both os.
To add Windows on a separate drive is easy. Only thing you might screw up with is the Windows drive MUST be formatted GPT if you are using Clover EFI to boot your OS X.
Easy method:
Connect Win-to-be drive and launch disk utility
Format the drive as if you were going to install OS X El Capitan
Shutdown, disconnect the OS X drive.
boot with Windows install media
at the drive selection screen, select the partition created with OS X disk utility and delete it
with the resulting free space hi-lited, click continue.
Windows installer will create the required partitions, format them and install Windows.
When install is complete, go to Microsoft update and run the updater until it tells you no more updates are available.
Set up your security firewall and AV app.
Shutdown, reconnect OS X drive and boot to UEFI/BIOS and make the OS X drive first in BBS boot order.
Boot Windows from icon named Boot Microsoft EFI from EFI.