It won't be able to test the drive -- leaving the question open as to whether the drive failed, or the system board did. All other diagnostics will proceed to their conclusion. If you have any doubts, pass on the deal in favor of a complete, working system.  
I am looking to buy a used XPS 13 9370 which has no SSD and for that reason the Dell Pre-boot System Performance Check displays a "Warning Message: Hard drive - not installed" which is understandable. But it also displays a "Critical Error Message - Testing completed. One or more errors were detected."   My question is: is that Critical Error Message a normal conclusion at the end of the Check as a result of the "Warning Message: Hard drive - not installed"?   What I am trying to find out is: if I install a SSD, will that Critical Error Message go away and will the laptop function normally?   Thank you for any input!
Critical Error Message - Testing completed. One or more errors were detected.