Have a 512gb Drive with mojave on it.. I re-formatted it to some APFS format and the only other drive i have with a system is with El Capitan.. Plug in the PCI drive and nothing.. Disk Utility doesnt see it.. how can i wipe that drive and install High Sierra on it when I cant even see it??
To mount an AFPS drive in El Capitan you have to use Disk Utility to re-Partition it as an HFS+ disk for access to it.
However if it is an NVMe PCIe blade drive, then only High Sierra has the drivers to access it, so it cannot be used with an El Capitan Mac.
Presumably (if you can mount it) when you begin to install High Sierra on it, the installer will re-format it to AFPS for High Sierra to be installed – that's what would happen if it were an internal drive, so I presume the same process happens if it is an external drive.