How so? Do you get any error messages? What version of OS X are you trying to use?
Without anything to go on, I'm guessing that it's because the hard drive is formatted as NTFS. Right click the external drive > Get Info. What does it say for format?
Your OS X NTFS driver only supports reading from NTFS partitions. If your going to share this drive with a windows machine you should format the drive with a FAT32 partition.
You can format it as FAT32 which will make it writeable by both OS X and Windows. However, doing so means that you have to format the drive which erases everything on it. You can follow the instructions here.
Remember - formatting will delete everything on the drive. Back it up first (save the contents elsewhere and move it back onto the drive afterwards).
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