Finally fixed my MacBook Pro!
After replacing the DVD drive with a second HDD and the primary HDD with an SSD I thought I was all set for a better computing experience. Unfortunately, after only a couple of weeks the funky hard drive performance I was experiencing before I replaced the HDD with an SSD returned. At the time I thought it was a problem with the motherboard, but eventually ran across the fact that it was probably the hard drive cable.
Straight-forward, painless, and completely solved the problem! I couldn't be happier. I had already upgraded to a newer MacBook for work, but now I've got my old 15" MacBook Pro back up and running. The new OS X El Capitan actually runs very smoothly on my SSD, making this an excellent, like-new test machine and toy for running Linux or Windows on VirtualBox. Thanks, iFixIt!
If the little pulsating LED light on the front-right corner of your mid-2010 MacBook Pro has stopped working, or doesn't work reliably any longer, it's probably the hard drive cable going bad. Replace the old HDD with a bigger SSD while you've got the machine opened up, and your MacBook will feel like a whole new machine.