Boots from USB but not from SATA

The backwards compatibility shouldn't be a problem in my opinion.
Sounds more like a failing internal SATA cable.

(Also, the bandwidths are 6Gb/s for the SSD and 3Gb/s for the MBP.)

 

Yep agree with above there have been many reports that the SATA cable is very sensitive and can need replacing along with the drive.

 

The 2010 MBP likely has the NVidia MCP79 SATA chipset - there are numerous SSD's that don't work well or at all with this chipset. The Corsair Force LS has a Phison controller. I was able to find a couple of articles on the Internet from people saying they had a problem with this combination - that's not much. But I also haven't been able to find articles from people saying they had the Corsair Force LS working in their MBP 2010 either. That it works with USB but not SATA, besides the cable issue, could be an indication that the drive (more specifically the controller) doesn't work well with the MCP79 chipset. But the Force LS doesn't seem to be too popular so there's very little information out there about whether it actually can work in the MBP 2010.

EDIT: If you can find a couple instances on the Internet of people who have the Force LS working in the 2010 MBP, you should probably try a new cable. A cable may work with the HDD but not the SSD. If you can't find anything on the Internet, you'd have to decide whether the cost of the cable is worth trying.

 

- Not necessarily. SSDs require more of the cable than HDDs. It's pretty typical that failing cables work fine with the original hard drive but not with newer SSDs.

If you are able to try out a different SSD internally, that'd be great - just to have some info on the possibly poor compatibility with the Corsair.

 

Agreed.