Your external HDD is most probably has a different file system if it was previously used on a Windows OS computer.
You can download a third party software utility that allows your Mac to read NTFS-formatted storage devices or you can simply type the commands using Terminal.
Here are some links that might shed some light on how to perform these:
http://appducate.com/2013/01/read-write-to-windows-ntfs-partition-on-mac/ http://computers.tutsplus.com/tutorials/quick-tip-how-to-write-to-ntfs-drives-in-os-x-mavericks--cms-21434
Your external HDD is most probably has a different file system if it was previously used on a Windows OS computer.
You can download a third party software utility that allows your Mac to read NTFS-formatted storage devices or you can simply type the commands using Terminal.
Here are some links that might shed some light on how to perform these:
http://appducate.com/2013/01/read-write-to-windows-ntfs-partition-on-mac/ http://computers.tutsplus.com/tutorials/quick-tip-how-to-write-to-ntfs-drives-in-os-x-mavericks--cms-21434