Thanks for the replies guys.
My main editing drive is a Lacie 5big ThunderBolt 2 drive, although I want something thats portable to use while I am not at home and have some time to edit.
I would really like one of these:
http://www.amazon.co.uk/Lacie-90004...&keywords=lacie+little+big+disk+thunderbolt+2
Although I've been eyeing up this, which costs a bit less and has almost double the storage!
http://www.amazon.co.uk/SanDisk-Ext...456170602&sr=8-2&keywords=sandisk+extreme+900
Last weekend I edited a small project using a Sandisk Extreme 500, which seemed to be fine, hence my reason for asking.
Which of the two above options would you go for??
Neither, but definitely the LaCie option if I had to pick one of the two. If I were were you, you're already just about halfway to a nice-n'-speedy solution tho'.
I like the LaCie drive, a couple of my friends have and use one, but it's not bus-powered, so you'll need a wall-wart for it. The main reason I don't like that drive is that we're all so close to TB3/USB-C solutions that are being announced every day. Not that you'll need the new stuff, but my vendors are trying to pawn off the "old stuff" as in TB2/USB3 to me every day to clear the way for the new stuff - the "old" but still serviceable stuff will be getting marked down big time in the coming weeks. That TB2 peripheral you're eyeing will be much cheaper soon, trust me on that (look at some vendor sites here in the US, like B&H Photo - so many items that are TB2 are "out of stock" now...). TB2 products are heading to the dustbin.
I don't like the SanDisk option. As an owner of the same rMBP you listed, you'll never be able to take advantage of its USB 3.1 Gen 2 interface & speeds. It's basically a USB SS device, with half of the potential throughput for us Mac users; AFAIK there aren't any adapters between TB1/2 and USB 3.1 Gen 2. TB2 carries USB 3.1 Gen 1 ("SS" or SuperSpeed") and TB 3 carries USB 3.1 Gen 2 but our Macs can't take advantage of that, even with a dock as we'd be limited to TB2 compatibility (hence, USB 3.1 Gen 1).
So, the halfway-there thing. One of my employees does our video stuff, and wanted more/bigger/better/much-much-faster. He's using a 1TB Samsung T1 drive. I loaned him my 1st-gen LaCie Rugged SSD (which has OS X 10.10 installed, and has both TB1 and USB 3 interfaces), and purchased a second, identical Samsung T1 drive. I then used Disk Utility on the 10.10 drive to put the two T1 drives in RAID 0, and the resulting RAID 0 array over the two drives installed in the two USB ports on his (and mine) rMBPs just frickin' screams. I'd suggest you try out the same, with the current and a second of the two SanDisk external drives - that's one heck of a scratch disk, and you'd be seeing both USB buses being utilized to near-capacity.
I have a newer LaCie Rugged RAID drive (two 2TB HDD drives, set to RAID 0 by default, but changed to RAID 1 by me) for field use. I used my 1st-gen LaCie Rugged SSD to change that RAID setup, as El Capitan's Disk Utility has RAID capabilities removed (insert grumpy comment here).
There's my 2 quid worth. Cheers!