Model Family: HGST Travelstar 7K1000
Device Model: HGST HTS721010A9E630
Serial Number: JG40006PGJL7XC
LU WWN Device Id: 5 000cca 6acc78a77
Firmware Version: JB0OA3B0
User Capacity: 1,000,204,886,016 bytes [1.00 TB]
Sector Sizes: 512 bytes logical, 4096 bytes physical
Rotation Rate: 7200 rpm
Device is: In smartctl database [for details use: -P show]
ATA Version is: ATA8-ACS T13/1699-D revision 6
SATA Version is: SATA 2.6, 6.0 Gb/s (current: 6.0 Gb/s)
Local Time is: Mon Jan 18 00:01:09 2016 JST
SMART support is: Available - device has SMART capability.
SMART support is: Enabled
The signaling speeds and the sata versions supported are indicated by different bits of the data return by the IDENTIFY DEVICE command. You can see that with smartctl --identify=wb /dev/sdX | grep -i sata.
These are the capabilities reported by the drive itself, so don’t mix it up with what is supported by your motherboard, which is CAN BE indicated by “current:” in the smartctl output (and dmesg as told in the other answer from @Jonno)
EDIT: Here is the exact spec file your drive claim to conform with: http://www.t13.org/documents/uploadeddocuments/docs2008/d1699r6-ata8-acs.pdf (ATA8-ACS T13/1699-D revision 6). You can see the two words in Table 29 — IDENTIFY DEVICE data. As you can see, SATA 2.6 and 3.0Gb/s are the latest/maximum defined as of this revision of ACS. So I guess your drive simply doesn’t completely conform with the spec and set the 6.0Gb/s bit, which is defined later, to 1 instead of 0.
The software that handles or manages a hardware controller is known as a device driver. The Linux kernel device drivers are, essentially, a shared library of privileged, memory resident, low level hardware handling routines. It is Linux’s device drivers that handle the peculiarities of the devices they are managing.