MacBook Pro (mid 2010) will not move past gray screen

I am running mountain lion (have been for about 6 months). On Friday evening, my system suddenly crashed and will not move beyond the gray screen with the apple logo. At first, I get the spinning circle, but then it disappears and the screen dims slightly and is stuck. I have reset SMC and PRAM, I have booted in safe mode, I have done permissions and disk repair, and I have reinstalled mountain lion. I don't believe it's a physical HDD problem because I am able to start from the second of two partitions and I can access my main partition through the second one. I have already spent time on the phone with apple support but they keep saying that it must be an HDD issue. I don't know what else to do! Can anyone offer any suggestions?