Precision T5600 8pin connector question

Hello, I really need some more SATA power connectors, and since I already have 4 drives in my configuration, I figured that I can use the 8 pin connector for that. I bought a 6 pin to 3 SATA connectors adapter cable, connected it, but got an error code stating "All PSU cables may not be properly connected to system board. (PS_ON asserted, missing a main power rail)". What am I doing wrong, and is it even possible to use the 8 pin port for that?

Best regards,

Georgi