OK...I've had success in changing the settings for installing Win 8 or Win 7 on the same system. I think I get it.
But every time I try to slave a Win 8 HDD to my system (either to a Win 7 or Win 8.1 system) I get an "inaccessible" error. Taking ownership does not work, since the options are all greyed out.
Last time, I finally ended up using an elevated command prompt to access and copy the files I needed from a Win 8 drive. Them oddly, after that, the drive became briefly accessible.
But when I tried to wipe the drive clean (using Disk Management) the options to delete partition, etc, were also greyed out, and it was inaccessible again - no "Format" options in My Computer, either. So I finally had to use diskpart to return it to "Unallocated Space". (Then Windows Disk Management worked.)
Does anyone know why this is happening, and if there is a less time-consuming workaround?