I have a 2009 Macbook running 10.6.8. My external backup disk failed and in desperation I bought a new one. But I find that the new disk (G-tech ATC, which might bounce better than the old WD) needs a later OS than 10.6.
I read bad things of Yosemite so tried to find earlier system.
Any OS later than 10.6.8 would probably do. Alternatively I could buy another drive - a temporary if pricey solution. I'd welcome suggestions if there's a way to get a legitimate upgrade from 10.6.8?

that's really odd to have a DRIVE that's not compatible with older OS. You'll get that with low level hardware like a SATA card that's physically connected to the computer but there should never be a problem with a standard peripheral via usb, firewire, esata, thunderbolt, etc. The drive and adapter at the other end of the cable should be totally opaque to the OS.

IMHO, if you are upgrading beyond 10.6.8, you might as well go all the way to Yosemite. I have a 2008 Macbook that runs Yosemite fine. I think the only reason not to upgrade would be if you have important applications that will not run in 10.10.

I resisted for years, clinging on to 10.6.8 until even the latest browsers were no longer supported. If you can't leave Snow Leopard, get a different external drive.

I have had zero problems using Yosemite, but did have to update several applications and lost Adobe GoLive.

I am a compulsove "early adopter" so take what I say in that light.

If you have any concerns at all about protecting yourself from exploits you should be running Mavericks (OS X 10.9) or Yosemite (OS X 10.10) as they are the only versions currently supported by Applle. And Mavericks will dropo off that list in a very few months when El Capitan (OS X 10.11) comes on line. Additionally more and more apps will require these verions as well.

Your biggest issue in upgrqding will be the necessity of bringing all your third party applications up to date as well. You are far enough behind the development curve there will be a bit of a change curve to cclimb, but it will only get steeper and higher as time and development goes on. Most of the problems people have had with Yosemite are the result of out of date plugins, kernel extensions, utilities, and detritus from older versions of OS X. On the plus side there a lots of folks around here who have recently gone through the upgrades and are willling to help you through the changes — and there are no few of those.

Of the options, the yet to be released El Capitan (OS X 10.11) is my favorite by far. (I am running it in a public beta.)

we FINALLY got you...

https://avgenes.files.wordpress.com/2012/07/nono.jpg

(I wonder if image embedding will ever work here again?)

…again???
As one of the moderators and one of the founders of FineTunedMac I can assure you that image embedding has never worked on these forums.

It's possible that the software that comes with the drive requires a later system version. But if you just connect the drive and format it with Disk Utility (don't use the manufacturer's software) it should work just fine.

I re(pre)sent that image.

Edit: Actually, I think it was Norm Crosby who said, "I resemble that remark".

I suppose you had to be there...

I took a look at your drive's specs, and this is, indeed, one of them:

.
But you bought yourself a second problem in the bargain!

Your drive has got only Thunderbolt and USB 3 ports, and since your Macbook has NOT got the former, and is limited to USB 2 throughput with the latter, you've boxed yourself into a relatively slow data transfer situation.

Your best bet is really to return the G-tech ATC and get a different external with FireWire capability.

Depending on your needs, you may be able to get what you need from OWC for around $125.

Edit: I'm not in a position to comment about Ira L's suggestion.

It's not clear to me that cyderman's new drive is the G-DRIVE ev ATC with Thunderbolt, since there is a version without Thunderbolt, but if it IS the former, then there's always the option of adding a Thunderbolt-to-FireWire adapter and accessing the Mac's FireWire interface that way...

...If cyderman's MacBook is not, in fact, the Late 2009 model, which lacks a FIreWIre port altogether.

Further...

The non-Thunderbolt model is strictly USB 3, so that limits cyderman as I said earlier (unless his MacBook is firewire-less), and while Apple's Thunderbolt to FireWire Adapter is only $29, its reviews are none too favorable...3 out of 5 stars, and it adds additional cost (and bulk) to an already costly solution.

In addition, there's something about Apple's adaptor being uni-directional, and something else about its delivering insufficient bus power, but my reading about the limitations has confused, more than enlightened, me.

I used the Firewire to Thunderbolt adapter only once, when I migrated from a Mac Pro to my iMac. There were no problems with it. Maybe it would have developed issues, but I can't say. At any rate, it was fine for me.

I think the problems are limited to external HDD connectivity; I don't recall reading anything negative about Mac/Mac connectivity.

Edit: I just remembered that one of the negatives was that the adaptor didn't fit snugly into its designated ports. Did you experience that?

According to moderator Dustin F at the G-Technology user forums, "[t]he G-Drive ev will not work on 10.6 as it does not contain the drivers for for the usb 3 controller used in the enclosure."

G-Drive ev & OS X 10.6 (Snow Leopard) compatibility?

Incorrect! As intimated by artie above, the adapter will allow a Mac with Thunderbolt to access a peripheral with FireWire, but not the other way around.

No, I didn't have that issue. It's possible that Apple revamped the connector before I bought it. I used it March 2015 and the adapter had been out for awhile, so perhaps it was tweaked a bit (?)