I bought the same drive several months ago and don't recall whether I had any trouble adding files, but then again it's my habit to reformat a drive for mac the first time I use it. I'd suggest you go to Disk Utility in the Utilities folder. As long as you haven't any files yet on the drive you can choose "erase" and reformat the drive. Mac OS Extended (Journaled) is your best choice if you only intend to use the drive with Macs. This is also a great opportunity to partition the drive if you think you'd like to have more than one partition.