I can open it and get files from it, but I can't delete files from it or add any more. The drive is probably formatted as NTFS (default Win file system). Without additional software, Mac OS X can only read NTFS.
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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. The Seagate would have come with Windows formatting.and. partitioning (Master Boot Record).
So when I get an external drive that will be used only for Macs, I repartition it (which also reformats it) as either Apple Partition Map (required to boot PPC Macs) or GUID. APM is the best universal choice for Macs, since it can boot both PPC and Intel Macs (though you have to clone Leopard onto it, you can't install directly.) If the drive is only for data, it doesn't matter which you use.
Sure you can do a simple reformat for Mac on top of MBR partitions, but then if you try to use the drive for Time Machine I seem to remember it will want to reformat it again. TM will format the drive, including the partition, properly. But then you can't use it with a Windows computer. If you can't copy any files to the new Seagate, perhaps it was formatted NTFS? I believe that's read-only on a Mac. If you need to have the drive be able to be cross platform you need to get a drive utility that will allow you to format the drive in FAT32 without the MSFT issued limit of 32GB.
I have many drives formatted in FAT32 that are WELL above the 32GB that they say is the limit (like 1TB). I don't know of a Mac utility that will do this but SwissKnife will do it very well on a PC.
You will loose some performance, but if you need to have MAC and PC read and write to the drive then its what you have to do. You will also loose the ability to use the drive as a Time Machine backup. Some food for thought.
You may be a mac only guy and not need this but i thought that this might help someone that is in a similar situation. Thanks for using Answers!