So I replaced the hard drive with a Crucial MX500 1TB SSD and the drive only operates in SATA 2 mode.  This appears to limit it to around 250 MB/sec.  There is no setting in the BIOS to change it.  Any way to make it use SATA III?
 

Same problem here with the Samsung 860 Evo 1TB SSD I installed. Samsung Magician software only showed 250 MB/s read and write when I ran the performance test.

I just found out that the HDD itself is only running at SATA II mode according to Crystal Disk Info.

This does seem to be a problem. Is there any BIOS configurations we can modify to maximize the transfer rate?

Of course, as there is no reason for an HDD to run at over SATA 2 due to it doesn't go that fast.  It is likely hard-coded in the BIOS to use SATA 2 on the secondary drive.

I would assume the it is actually a SATA 3 interface which is limited in some way, do they still make SATA 2 interfaces? lol

starrbuck
Of course, as there is no reason for an HDD to run at over SATA 2 due to it doesn't go that fast.  It is likely hard-coded in the BIOS to use SATA 2 on the secondary drive.

So you mean the controller should automatically switch to SATA III when an SSD is attached?
 
I have no other laptops to test, but on my desktop I'm seeing Transfer Mode of "SATA/600 | SATA/600" for both HDDs and SSDs.

Yes my SC17 the HD was upgraded to a SSD and I get SATA3 speeds, so EVGA really skimmped ou on including ony SATA2, as if no consomer planneld on upgrading eventually or immediately. Sure PS4 is Sata 2, who cares, but a premium SC15, that should at alt least have a 1070MQ by now, come on.   Eitheir way, Sata2 as a backup is plenty fast but very strange at this time and age  for a brand like EVGA, or any brand for that matter, even some of the newer consoles are now sata3.

SC15 specifically mentions 6G so....
https://www.evga.com/products/pdf/516-34-1833.pdf
so try upgrading RST driver. 
https://downloadcenter.intel.com/downloads/eula/27681/Intel-Rapid-Storage-Technology-Intel-RST-User-Interface-and-Driver?httpDown=https%3A%2F%2Fdownloadmirror.intel.com%2F27681%2Feng%2Ff6flpy-x64.zip
In Bios set Sata to ACHI, if not available, then unfortunately only by unlocking by Bios. It might be set to to IDE.
YOu can an also try to restore BIOS setttings to see if t get goes going at SATA 3 instead of 2.

vistarshook
SC15 specifically mentions 6G so....
https://www.evga.com/products/pdf/516-34-1833.pdf
so try upgrading RST driver. 
https://downloadcenter.intel.com/downloads/eula/27681/Intel-Rapid-Storage-Technology-Intel-RST-User-Interface-and-Driver?httpDown=https%3A%2F%2Fdownloadmirror.intel.com%2F27681%2Feng%2Ff6flpy-x64.zip
In Bios set Sata to ACHI, if not available, then unfortunately only by unlocking by Bios. It might be set to to IDE.
YOu can an also try to restore BIOS setttings to see if t get goes going at SATA 3 instead of 2.

I went into the BIOS settings and figured out the option for changing controller speed to SATA III just before your post .
 
The controller it self is capable of SATA III but the default (and hidden) option in the BIOS limited it at SATA II.
 
Yes, you do need to unlock the BIOS to change that configuration, although the edit is quite simple.
 
=============== DISCLAIMER =============
The steps below may result in possible hardware failure in
rare occasions. By following the instructions, you are fully
responsible for the outcome.
=====================================
 
Grab the tool called "AMIBCP" (from Google), open the BIOS file (in my case SC173106.bin, even though it is actually v1.06 for SC15), and modify "Access/Use" of "  -> Setup -> Advanced -> Hide Item" to "USER". (as shown below)

 
Save and flash the modified BIOS file and reboot into BIOS (by pressing F2 during booting). Follow the screenshots below (disable "Hide Item" to reveal all hidden items):
 

 
Save & Reboot and now your HDD/SSD should be able to reach full SATA III speed!
 
P.S. Not all hidden items work correctly, so tinker at your own risk. If you are stuck at a black screen after changing some settings, use the reset button at the bottom center of the laptop to reset all settings to default.

Why are the bios so limited in the first place? You basically have no options to change by default. This is something Apple would do lol.