At SL you can get (4) 750GB hardrives. You can talk to them to get a larger setup, but i think that 3TB is going to be the largest you can get from the mainstream providers without getting a custom quote
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IMHO, with that kind of hardware, it may be cheaper and better to colocate.
So my reasons for this are two-fold. I'm just about to provision an 8TB Jetstor san, and 4 servers to a new customer. I'd like to know where my pricing is compared to anybody else that might have public pricing on such a service. Also, I've got a 4TB backup raid here I'd like to backup remotely. You're right, though, it'll probably just make more sense for us to colo it.
I would recommend colocating. If you for some reason do not want to colocate, nobody is going to have this server as a default package. Therefore, you will have to go custom with a well known dedicated server providers like SoftLayer.
Because its so unique, the provider would probably have to custom build the server for you and that means large setup. It may, in my opinion, be cheaper to split it up amongst several servers that still have large capacity (1-2 tbs)--as then it wouldn't be such a rare order/present a lot of risk to the provider if you back out after 6 months.