Hello nickel3,   We have seen this if the controller card is totally or partially disconnected from the backplane.   Shut down, remove power. Loosen  the blue screws for the front controller,   reseat the controller  and tighten the blue screws. Reseat SL (slimline) cables, and cables to backplane on both ends.   Dell EMC PowerEdge R750xa Installation and Service Manual [URL]  
Hey everyone, I have a PowerEdge R750xa which was running fine until recently. After briefly opening the chassis yesterday to simply check for the GPUs, it won't boot up anymore. All internal devices appear to be connected correctly. When plugging in the power supply, the front panel indicates "Initializing...", then just briefly shows the message "Chassis is open" and finally only shows "Unable to power on the host because an SL cable is connected to an unpowered device". I interpret this message as some device is connected through data cables but not to any power supply. However, upon multiple inspection I cannot find anything like this... Can anyone help me out with this and push me in the right direction?  Best regards,Manu
Unable to power on the host because an SL cable is connected to an unpowered 
device