External Storage
How to select the right external storage
The quick answer is to confirm the external drive is large enough to store all your data. New cameras and video cameras are increasing in resolution and their file sizes increase as well; not to mention that most people store their whole music collection in iTunes. The average life of a spinning hard drive is about 5 years, so make your best guess as to how much storage space you will need 5 years from now before you need to buy your next external hard drive.
The detailed answer involves physical size, flash memory vs spinning disc, and file system type. Any drive can be reformatted to meet the needs of the situation so don't worry too much about the "file system" of the drive. Most (non Apple drives) are FAT32 and work on both PC and Mac.
If you have a Mac, only want to use it on a Mac, and don't want to worry about formatting the drive, you can shop at the Apple store for your drive
Portable external drives: http://store.apple.com/us/browse/home/shop_mac/mac_accessories/storage?n=portable
Flash drives: http://store.apple.com/us/browse/home/shop_mac/mac_accessories/storage?n=flash
If you would like to use the drive on both Mac and PC, the drive will need to be either partitioned into both HFS and FAT32 or should be reformatted into all FAT32.
If you use Time Machine or place your iMove database on your external drive you must have an HFS formatted partition.
Learn more at Apple Good to Know Guide on Storage
http://store.apple.com/us/browse/guide/storage?mco=MTM3NTIxOTY
Partitioning a disk into several volumes and formatting
Using Mac
http://docs.info.apple.com/article.html?path=DiskUtility/10.5/en/duh1012.html
Using Windows
http://www.bestarticles.com/computers/how-to-partition-an-external-hard-drive-553.html