Asus TS100-E7 - Manual (Page 49)

ASUS TS100-E7/PI4 3-9
2. Hard disk activity LED connector (4-pin HDLED1)
This LED connector is for the storage add-on card cable connected to
the SATA or SAS add-on card. The read or write activities of any device
connected to the SATA or SAS add-on card causes the front panel LED to
light up.