I have a 1Tb external HD with 2 partitions. I remove the first one to extend the second to fill the entire disk, but find that I can only extend to the end of the disk, and the unused space is in the begining. Do I have to erase the whole disk and repartition it? - It's not a master partition, just Data, formated as MacOS extended.
What you've discovered is true -- Mac OS X's support for HFS+ volume resizing allows the end of the volume to be extended, but it doesn't allow you to move the beginning of the partition.
If the free space that used to hold the first partition is big enough to store the contents of the second partition, then I suppose you could save yourself a reformat by creating a new HFS+ partition in the now-free space at the beginning, then copying all your data over there, then deleting your second partition and resizing the first to fill its space.