Honestly, at this point, if I were you, I'd be pricing replacement drives because that one sounds like it's in the very early stages of failure. Five or six years out of an external HDD is an astonishingly good run however. Usually externals don't make it more than 2 years.
You could try, since you have backups of everything on the drive, wiping it clean and recreating the partition structure, then restoring the files from your backup, but I doubt that will solve the problem. It will probably just move the problem to a different partition. Still, you have backups, there's at least a chance it will resolve the problem, and it is a free solution. If that doesn't do it, then you can start looking for a new external.
I have a PowerMac G5 running 10.5.8 with two external hard drive. One
of them (carrying the brand IOGear and identified on my System Profiler as
being manufactur3d by Prolific Technology Inc) has been operating just fine for
almost as long as I've had the computer (early 2005). It is partitioned into 5
volumes that have been serving faithfully as storage for my many files. About a
month ago, I realized I did not see the volume icon for one of the partitions on my
desktop. I checked it out in my Time Machine files and it's there along with
everything else and getting backed up regularly along with the rest. At the
time, I restored that information into a folder on my desktop. I can now access
information from it, but I cannot save to it. When I try (from Word), I get a
message saying that Word can't save or create the file... the disk may be full or
write-protected... and to try one or more of (a) freeing more memory, (b) making
sure the disk is not full (that volume, like the others on the partitioned
disk, is less than 1/3 full), write-protected, or damaged. I have run Disk
Utility a couple of times on it and it says that volume and the others on the
drive are all just fine (okay). However, it identifies that partition as not
being mounted (the other four are mounted and working fine), and when I try to
mount it through Disk Utility, it says it can't and that I should try repairing
it... As I said, when I check it to see if it needs repairing, it says it's okay
so it doesn't repair. Also, I am unable to do a "Get info" on it from the
Finder because I have nothing to click on to then get the "Info" on... hope that
makes sense. Finally, my question (probably obviously) is "How do I get this
partition mounted so I can use it again????" Any help will be very, very, very
appreciated. (P.S. I've tried all the 'stuff' like turning the drive off,
letting it rest, turning it back on, and all that jazz.)

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic