partition external disk on Mac

1) Open the Disk Utility

2) choose the disk that you want to make partition

   --- back up the data before doing any partition

   --- If there are several partitions existing in the disk, choose the whole disk, not a partition, if we want to have the tab Partition appearing in the window

   --- Choose Erase tab and OK

   --- choose Partition tab: decide how many partition you want to make, then for each one you give the partition name, partition size, the format, as well as the GUID (mac osx) or MRB(windows) in the Option. Choose Apply.

 3) then the disk should be partitioned according to your need.