I'm about half way through, http://www.dedoimedo.com/computers/gparted.html
"GParted Partitioning Software - Full Tutorial", so at this point I have learned how to recognize drive letters (hd, sd), partition numbers (1-5+) and understand physical and logical with respect to partitions and extended partitions.

I have reconfirmed with an invoice and reading off the drive label attached by the drive manufacturer, that I have a 500 Gb (464 GiB) external drive. I'm running this on Firewire using a driver I got off of elrepo.org. When running gParted at boot with a Live CD, it looks like I have 750 Gb on this drive.

I can select /dev/sda, /dev/sdb and /dev/sdc in the upper right drop down menu.

/dev/sda 93.16 GiB
/dev/sda1 flag: boot, 101 MiB, ext3
/dev/sda2 flag: lvm, 93 GiB
unallocated : 2.4 MiB

/dev/sdb 189.92 GiB
/dev/sdb1 flag: boot, lvm 189.92 GiB)
unallocated : 6.64 MiB

/dev/sdc 465.76 GiB
/dev/sdc1 flag: EFI, 3.09 MiB, fat32
/dev/sdc2 flag: Martha 14, 465.44 GiB, hfs+
unallocated : 128.02 MiB

Add those up, (93.16 GiB+ 189.92 GiB + 465.76 GiB), and you get 748.84 GiB. I am really confused! Can you help me understand why my ~750 GiB external drive is really a 500 Gb (464 GiB) external drive?

You seem to have three drives, the first has two partitions: ext3 (linux) and a lvm (linux volume manager), second has a lvm partition, and third (sdc) which should be your external drive, has two partitions: fat32 (windows) and hfs+ (apple).

Is this your system? if you did that partitioning scheme you should know what you did there and why.

The other devices are also counted. It seemed that you have three devices (two besides your external).
Is that what you're asking?

My total system configuration is hda and hdb internal drives, and this external drive that I'm trying to figure out. The external was last used with Apple OS 10.6.

Oh! Now I get it... it's assigning my internal drives as sda and sdb, even though CentOS, when running, assigns them hda and hdb. It's reading the external as sdc. My confusion was, as per the reading I was doing, sd(a-z) is a SCSI/SATA/Firewire drive designation, not an IDE drive designation. GParted Live CD assigns all drives (IDE, SCSI/SATA/Firewire) and presumably Raid as well, the sd(a-z) designation. Is this the right interpretation?

And sure enough, the size reported for hdc is 465 GiB, which, when converted, is 500 Gb. Just what I thought I should have.

That's right! It seemed that you have made a confusion by not noticing the number of drives connected.
Mounted or not, they are still counted.