I am trying to install the Snow Leopard upgrade on my MacBook (Mac OS 10.5 Leopard), and I can't seem to do it because it tells me to change my partition scheme table to GUID. I went into disk utilities and clicked on the "partition" tab, but I can't click on the "options" button. I cannot erase my hard disk either. What could be the problem? Just in case this is important: I changed my 160GB original HD to one of those WD passport HDs. So now my internal HD is the 500GB WD passport HD. Help Please! I already backed up my HD onto an external HD using Time Machine, but I cannot erase my internal (WD) HD!