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.

iMac (19,1, 3.1 GHz i5, 12.7.4, 40 Gb RAM); MacBook Air (1.8 Ghz, 8 Gb RAM, 10.14.6, 256 Gb SSD) Vodafone router and Devolo Wi-Fi Extender, Canon TS8351 printer/scanner.