I have a 2010 MBP that I am selling. ~3 weeks ago, I upgraded that machine to Lion. Last week, I bought an iMac. I did the Migration Assistant thing and transferred everything from the MBP to the new iMac. Now I want to wipe the HD on the MBP clean and put a fresh install of Lion so I can sell. So, I restart the MBP, I hold down CMD+R during the boot up, I select Disk Utility, and I select my drive and click erase. I get an error that says "Couldn't unmount drive." Not sure how to proceed.