How to format an external drive (Mac):

What if you need to use the same external hard drives with both types of computers? If you set the external drive up using NTFS then the Mac can’t write files onto it. Rats. If you set the drive up using OS X Extended then the PC won’t even recognize it. Double-rats. So what to do?

Read full content : How To Format an External Hard Drive or USB Flash Drive for Mac - Toptenbiz

The very best solution that I have found to this dilemma is to avoid this whole problem. If you can use one external hard drive for your Mac and a separate drive formatted for a PC when your working with a Windows computer. But if you must use the same drive with both types of computer then you need to install some additional software. PC users who face this problem should look into a super cool program called Mac Drive 9. With MacDrive, your PC can both read and write to an OS X Extended hard drive!

Mac users who find themselves in a similar position should look into a program called Paragon NTFC. Like MacDrive, this inexpensive utility gives a Mac the ability to both read and write to an NTFS hard drive. Once I have MacDrive installed on my PC, I can use an external hard drive that has been formatted using either the NTFS or OS X Extended system. Likewise, with Paragon NTFS for Mac OS X installed, my Mac suddenly works with either drive format. Problem solved!