I'm thinking... with so many disks is it worth, cost-wise, to go to storage spaces and handling such problem in the windows stack instead of buying a respectable external array with built-in virtualization and just present simple LUNs to the backup servers?
you have also the flexibility to use such provisioned space from multiple servers in case, simple hand offs in case of faults etc. I don't know if the cost of such space provisioned via direct attached disks is so economic, especially considering all the variables (flexibility, the external space "survives" the lifecycle of the server itself etc.etc.etc.), and especially if you have already a fabric in place.
just a consideration regarding the tradeoffs of the solution...