Account closed as requested
I have similar experience, a few months ago I had to disconnect the rest 5 disks to be able to install Windows 10. I found an installation logfile somewhere on X: (virtual filesystem used only during installation, it is necessary to push Shift+F10, run Notepad from command prompt, select File-Open and then find the log) which said something like "the selected disk is not the primary boot device", I do not remember the exact sentence. IMO the problem is not number of disks (even it is possible too) but disk sequence, because for some reason it is presented differently in BIOS and in Windows - Windows installation probably wants to use the very first disk. If you have the target disk for Windows 10 connected to add-on SATA controller card instead of motherboard (same as I have), you probably have the only one option - to disconnect all other disks temporarily.