on the right of the psu there's a black plastic cover, then there are 2 hdd caddies. If you remove the black cover, underside you should see the sata connectors
Is there any way I can add HDDs to this?  I see only one connector and don't want to add an NVMe or another SSD one.  I have 2 external usb drives and would like to install one or both internally.  
DELL PRECISION 5810 Workstation how to add hard drives