https://support.hp.com/us-en/document/c05371240 35w i5-6600T My low spec models have the nvme slot occupied with a 256gb oem consumer samsung drive, the pm981, intended to be the OS drive. The upgraded one comes with a 2tb enterprise sata drive, the pm863. There’s no other internal storage expansion. Rams upgradable, 8gb in the low spec, 16gb for the upgraded one. Mix/match random generic 4/8/16gb sticks, same model whenever needed.