The mounting issues are caused by a defective phone or a bad setting within the phone, not the computer.
Last night, as a test, I hooked my wife's Epic into the our iMac and our Windows 7 laptop. The phone sync'd just fine with both computers recognizing it as a USB drive.
Then, without changing anything, I tried to do the same with my Epic. Neither computer would recognize it as a USB drive. The iMac thought it was a networking device and the PC acted like it was installing drivers but then ceased to recognize it at all. To rule out a bad SD card, I took the card out and plugged into an SD reader; that worked just fine.
Given this, the problem is clearly with the phone. I'm taking my Epic back to the Sprint store today, which wouldn't be so bad except for the fact that was a warranty replacement for the Epic that broke last week. Needless to say, I am not a happy Epic owner.