I am trying to expand my HD drive currently at 124GB. I have reduced HD2 by dragging. Tried to drag and extend HD but it does not work. Funnily enough I can reduce the size of HD but cannot expand it beyond 124GB as orginally partitioned.
I don't have enough hard drives lying around to check this for you, but my recollection is that if you have unused space on a GUID-partitioned drive, you are free to extend the last partition to take up that space. This is assuming you're using Disk Utility on an up-to-date Mac.
Say you have, on a single drive, two partitions: HD1 and HD2, and some unused space, in that order. You can extend HD2 because it is contiguous with the empty space, and you can shrink either partition as long as they are not at capacity.
However, you can't extend HD1 because HD2 is the next segment on the drive. Extending HD1 would require HD2 to shift forward x-million blocks on the disk, which cannot be done in one step.
What you might want to do is delete HD2 so as to make HD1 contiguous with unused space, then extend HD1 to the desired size, and finally re-add HD2 from the remaining unused space.
You will lose all data on HD2 in the switch, so back it up (do that anyway) to an external drive or to HD1 and then restore it once everything is set back up.