Use those kexts to avoid "Still waiting for root device" on Dell Optiplex 390
Tested on Hackintosh Hign Sierra Zone
Place to Extra of USB Drive and boot installer in Legacy Mode with -f -v
Then after installation boot USB Drive again with same boot flags (but choose your HDD, not installer) and install these kexts to /L/E using KextBeast or any other tool that can install kexts to /L/E