How has the External HDD failed?
I presume its a USB expansion drive. We've had a couple of these, and a couple of failures. Inside the casing is usually just a standard 2.5 (lap-top) or 3.25 (desk-top) IDE Hard Drive. Casing has an interface to power up the disk and talk to the IDE interface on the disk and USB plug on the cable.
On the larger 3.5" HDD's we've had a couple have had the power adaptor go on us. Easy enough to get a new power pack of e-bay for £2.99 or something... and back in business.
Another, the interface has bludgered... too much plugging ion and out to shift it between computers..... Cracked the case, took the HDD out, and plugged it into spare bay on the Desk-Top... then ordered a new enclosure with interface for it from e-bay for I think it was £7.
That disk, after whet two recoveries, eventually did 'die' horibly. The actual heads started clicking as the on-disc control electronics fried. We put this down, to the fact that in the little plastic enclosure, and powered up the whole time, being run not as a back-up or part-time drive but main media storage on the computer, it just got too hot, too often to last, without any form of heat-sink or cooling fan.
'Recovery' via specialist was a question of stripping the drive, and rebuilding it. If the interface electronics, swapping the flexi-strip between the interface sockets and heads wrapped around the metal chassis for one from another, identical drive, or taking the actual disc-stack out and popping them into another identical drive.... NOT easy, and its expensive as you have to add the price of another identical HDD to replace the broken bits to get the data off the discs to cost of time to do it...
As drive didn't have anything that was irreplaceable on it... it simply got binned at that point.
In between times though, it did manage to get unintentionally re-formatted, when her-nibs the 'Format-Queen' had choked her C-Drive with 'stuff' and the computer decided it didn;t want to recognise it... so she stuffed the Windows disc in to the CD and pressed 'Y' when it asked her if she wanted to format the Hard drive... the only one it found..... on the USB cable!
There was some stuff worth recovering on it that time round.... only real bug-bear of it was the disc was a 1Tb drive, and using free file recovery software, we had to have at least that much 'spare' space on a computer to put it all! And we didn't! I ended up clearing down my Lap-Top with a 320Gb HDD, then telling recovery software to only recover individual file types; mpg or jpg, or whatever, so as to limit how much it pulled off the disc in one go, in hope it didn;t swamp the lappy! then burned that to DVD, soing successive 'sweeps' to get all file types we wanted off it!
So... first port of call. HOW has your HDD failed? And can you get it up and running cheaply and easily, cracking case and plugging it into a desk-top directly?
If so, may be salvaged either with new enclosure or just a new power-supply; and if a computer will still talk to it; even if the disk Is corrupt, file recovery could still let you pull data from it.
If not, and it is toast.... its toast. And it would have to be rebuilt, expensively to get it to talk to a computer, and recover anything recoverable left on it.
Is whats on there worth that kind of cash, is the real question? And we cant answer that. Only you can.
Meanwhile; Extrenal Drives are NOT the most reliable.
The external supply 'desk' drives, are probably the most at risk. Powered from seperate power adaptor, when you turn OFF the computer, the Hard drive MAY still be powered up, even though its not talking to anything through the USB. As mentioned, without heat-sink or cooling fan, they aren't best cooled to begin with, so leaving them constantly switched on, isn't going to improve thier odds of long term survival.
IF you use them; back up to DVD, and make sure that when they aren't in use, they are unplugged and unpowered and not cooking themselves on a low heat.
DVD? I remember when CD's were 'invented' and a couple of Radio 1 DJ's picked up on their allegded 'indistructability' and went out and played frisby with a couple in the park before playing them on the radio! No... they aren't are they!
However, optical discs are pretty hardy; and write-once read many, once you have burned data to them, as long as the burning software hasn't used an unusual or incompatable burning algorythm; its normally pretty safe.
I have had a few discs, and I have been burning them since I got a very early Philips TWO SPEED CD-Writer, when they were first launched, for some stupid amount of money more then my car at the time was worth! I've had a few discs that have gone duff over time. Couple through physical damage; others through software compatability; most common, probably 'multi-session' burns, where I have only put a half discs worth of data on the disc, and left it 'open' to add more later; that disc has only been readable in my CD drive, on my computer, and on the same instalation of OS as was on it when I opened the disck to write! So these days I always burn and close. Gauling to only put perhaps 500Mb of data on a disc that could take 4,500Mb... but, what the heck; 4.7Gb DVD's are the same price as 700Mb CD's now! Just bought another cake, £15 for 100..... they are 15p each! (Jeez I when I got that two speed CD Kodak writable discs cost me £3.99 EACH!)
100 x 4.7Gb discs for £15 is 470Gb, or 3.2p per Gb. Compared to about £75 for a 1Tb HDD, which gives you, 7.5p per Gb, its about the cheapest storage you can get.. though chopping up a hard drive to fit of hundred discs, you probably loose a bit from only being able to part fill each, either because of file-sizes or simply convenience.
And it IS pretty reliable. Once on the disc, as long as you do store them reletively carefully, and put each into a protective slip or Jewel case, I use PVC sleeves, something like 200 for 99p of e-bay or something daft... adds not a lot to the cost per Gb anyway... then archive in dish-washer tablet boxes... Hey! they would just get thrown away, and they are a convenient size to take about 30 DVD's!
So, when you wipe-down camera's SD card; what do you have on yours? Mines 16Gb. Four DVD's at most, if the card's full. Archive straight to DVD, mark the disc by Wipe-Down date. Sleeve and box. Whatever happens to hard drives after ought not matter much!
HDD space is then 'working' instant access storage; and an external may be a useful way to get large data capacity to hold a lot of photo's to look at.... but, how much do you really need?
I have over 65,000 images on my PC, pretty much my entire archive collection; They take up about 100Gb of a 500Gb 'slave' or second HDD, in the Desk-top. And that's why I put it there. It has a heat-sink and fan on the bottom so that it is kept cool under frequent or continuiouse use, to prolong its service life, and plugged directly into the motherboard via IDE cable, big files load fast without lag. and I have all my USB ports free for memory sticks, scanners, Car-Readers or cameras!
Wouldn't be so convenient on a Lap-Top; that only has room for one internal Hard-Drive, and at 320Gb, taking 1/3 of that for existing pictures wouldn't leave much for anything else... which is why I have a Desk-Top for photo-viewing or editing. I can still access my Photo's from the lappy via home-hub off the desk-top, and could load a few to it if I wanted to take the lap-top out the house I suppose... but then most photo's I may want to show any-one are loaded at web-res to Face-Book or Photo-Bucket anyway.
If I only had a lappy, then an external HDD might be only real option... and could be OK... but as 'working' mass storage, not archive space.... master files still archibed straight to DVD at card-clear time.
Bottom line; magnetic memory is NOT permenent long term memory. Its designed to be written and over written; its not write once read many. Optical, CD/DVD is WORM write once read many memory you cannot over write, and that doesn't have metal moving against metal to review it every time you access a file; only light. It is the medium DESIGNED for long term archive storage. External HDD's aren't.