This is just the beginning of a work-in-progress to fully utilize the extra storage available on the 200+ workstations in my department. Updates will be posted regularly (hopefully). In the end, I hope to present a step-by-step guide to fully utilizing the untapped storage potential of departmentally owned workstations.
Environment
200 PCs (staff and lab machines) running Windows XP Pro, all with at least 40GB free.
Goal
Utilize more of the storage we have already paid for. Most workstations are online 24×7, and have unused disk space (20GB to 200GB). I would like to pool the storage in a highly redundant fashion (ex: 4 machines contain the same data so 1 can be powered off with no impact on data accessibility) to present a large block of usable space.
Possible technologies to use
· DFS
· iSCSI+ZFS
· iSCSI+software RAID
Possible uses of combined, redundant, pooled storage
· CIFS shares
· VTL storage
· large temporary storage for student use
· large DB backend (probably not a great use for this, but given the number of spindles, it has potential)
Benefits
· highly redundant
· LOTS of spindles
Possibly helpful links
http://www.faqs.org/patents/app/20080320097
http://www.datacore.com/
http://msdn.microsoft.com/en-us/library/bb524801(VS.85).aspx
iSCSI Target software for Windows
http://www.iscsicake.com/
http://www.emboot.com/products_sanFly.htm
http://www.kernsafe.com/Default.aspx
Nothing to see here 
I have wanted to do this for years. My direction of thought has always been towards a modified P2P client that would just use a percentage of the currently unused storage on our workstations and integrate with Windows, etc. to allow the mapping of drives to the storage.
Have you looked at branchCache? It can be useful though not exactly what you need…
http://technet.microsoft.com/en-us/library/dd637784(WS.10).aspx
I honestly had not heard of that before today. Our lab computers currently run Windows XP, but we definitely plan on upgrading to Windows 7 soon. The “Distributed Cache mode” looks very promising. The next hurdle (after I get it installed) is making sure data on lab machines isn’t accessible by users. If this can work with EFS or similar encryption, BranchCache may be the answer I’m looking for. Thanks for the feedback!
I have not found much technical details about BranchCache – I actually learned about it when I did the beta of the Win 7 MCTS exam ! (not a good way to learn about stuff!)
And indeed I wonder how stuff is encrypted, I mean, EFS or not, it needs to be encrypted else it would totally defeat the point of NTFS access rights on the shares you replicate..