It's very common to use an external firewire drive for video storage on a non-linear editing system. However, these drives are typically formatted FAT32 by drive manufacturers.
The FAT32 file system can create problems for video makers because it limits files sizes to no larger than 4 GB - about 17 minutes of DV video. (You can learn everything you ever wanted to know about FAT file systems by clicking here.)
The answer is to reformat using the NTFS file system the first time you connect the drive to your computer.
Important Note - Reformatting erases all data from the drive. If you have already stored data on your drive, you need to back up that data BEFORE reformatting.