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Using Lion’s new Recovery partition and its pre-installed tools, don’t try to erase the main hard disk of a MacBook Pro. You can’t do it. The drive won’t unmount to perform the partition or erase commands in Disk Utility.

It’s because the Recovery partition is “part” of that main hard disk and therefore the hard disk is technically in use. Disk Utility won’t let you partition or erase a disk that’s in use.

Here’s a word of caution about this. OWC blogs about this issue of cloning Lion-based hard disks due to the Recovery partition. Seems like the best bet is to create a bootable USB Flash drive, start from it, and perform the Disk Utility actions.