you should think about formating your Ext. HDD to FAT32...it will allow you to read and write on both Mac and Pc, and it should be a little more compatible i guess. While Mac can READ NTFS partitions, im not really confident on its abilities. I have an Ext. HDD (which was original NTFS), and it was kinda weird, it would pop up in the desktop, and after a while, disappear (I know some HDD shut off to save power, but they would still show on the desktop, this one however, just dissapeared, I had to manually turn it off, and back on again). Once I formatted it to FAT32 it worked wonderfully.
Anyways, if you dont have a bunch of data, you should copy all of it to another HDD, and format it to FAT32....just a suggestion though.