Assuming this external HD is a USB hard drive, after plugging in the drive have a look at the end of the /var/log/messages file;
tail -12 /var/log/messages
Then do something like;
mkdir /mnt/External ;got to have someplace to mount the device to, only need to do this once.
mount -t auto /dev/sdXy /mnt/External -rw
Where X is the hard drive (sda, sdb, sdf for example) and 'y' is the partition one wishes to mount. The output from the tail command should provide the information you need, some more examples;
mount -t auto /dev/sdb1 /mnt/External -o rw
mount -t ntfs-3g /dev/sde1 /mnt/External -o rw
If the filesystem on the external HD is NTFS then have a look at installing fuse-ntfs-3g, you can find the rpm package at the RPMforge site and instructions on how add this repository here;
http://dag.wieers.com/rpm/FAQ.php#B
Also see:
http://wiki.centos.org/TipsAndTricks/NTFSPartitions