create new partition without deleting current one

if you select the current partition in the partition tab click on + below it to split it in two. note this only works if the partition is formatted mac os extended and the drive is partitioned with APM or GUID partition scheme. make sure to leave plenty of free space on th existing partition or the operation will fail.
enter "create new volumes" in Disk Utility help for more details.

You should be able to use Disk Utility - select your drive, then the Partition tab.

Provided your existing volume isn't full, you should be able to drag the handle in the lower-right corner of the partition view to resize the existing partition to make it smaller.



Once you have some unused space you can click the + button to add a new partition in the vacant space.

sorry, i can't explain that. was the drive formatted on a mac running leopard? sometimes drives formatted from tiger won't let you split volumes. but in any case, if it won't let you use + then I don't think either mine or Camelot's method will work (you can try his) and you won't be able to do it with DU without reformatting the whole disk. a 3rd party program like ipartition may be able to do it. but I would suggest you simply clone the drive to another one, repartition this drive and clone the clone back.