Wild SATA data and power connectors appear! This pleases iFixit.

If you were worried about storage capacity, worry no more. It seems the Steam Box is prepped for a second hard drive to store all your games.

This particular Steam Machine has a 1 TB Seagate ST1000LM014 laptop SSHD (Solid State Hybrid Drive). It's a 2.5" SATA III 6 Gb/s, 5400 RPM SSHD with a 64 MB DRAM cache and 8 GB MLC NAND Flash.

TL;DR This is a one terabyte platter drive paired with an 8 GB SSD and some clever software that keeps your most-frequently used data on the super-fast SSD.