A warning: if you have external hard drives formatted under NTFS, you may need to reformat them as FAT32 or MAC OS Extended filesystem. OS X won't write to an NTFS drive.
This was a problem for me because my external HD has all my iTunes music. I used a great free utility called fat32format - it formats FAT32 drives > 32GB.
You can find the utility here.
I think you can format the external disk specifying an MS-DOS filesystem using Disk Utility, but I didn't try that.