You might know M.2 as NGFF, the ‘next generation form-factor’ for plugin modules on laptop and small-form-factor motherboards.  You can find out more here but in short, it’s either a SATA or PCIe interface to modules like SSD, LTE and the like. MyDigitalSSD kindly sent a half-height M.2. SATA over along with a $199 Chromebook so here’s the how-to, video and results of the Acer C720 Chromebook upgrade.