To create a partition in Mac OS X please open Utilities menu -> Disk Utility and do the following changes:
5. Press Apply to finish the operation.
This step is not a requirement for most drives but is necessary in following cases:
1. If you are going to connect this drive to a computer running Windows XP or earlier editions or media devices such as media players and TVs. They only support MBR.
2. In case your device is assigned as external drive (almost all the flash drives and some external HDDs are). This is required because Windows systems only read the first partition on an external drive. And if it’s a GPT disk (which is a default setting for formatting in Disk Utility) it has a small system partition in the beginning. Thus Windows reads it and skips all other partitions, although they all can be seen in Disk Management.