GUID partition scheme If I try to change a partition scheme of a 4 partition HDD from Apple to GUID, iPartition warns me that 'system partition' will be erased. What does that mean? Thanks.
If you have several partitions on that disk, then it is telling you that you will lose the partition that has the Mac OS system installed on it, including all the data on that partition and the Mac OS system you have installed on there.

My advice is that if you don't understand the warnings, then you really should not be in there - it really is SO easy to click a few buttons and lose all your data.