If you have lots of storage to spare, one way is to share it out via a file share. This is useful on a file server level. What about sharing spare disk space at the storage level? There are solutions available, mainly in the Linux word, Openfiler project etc. What if you have a Windows server? One possible solution is the Microsoft iSCSI target. Yes, you read it correctly. It existed for the Windows 2003 version and extremely expensive.
Browsing through the old posts of SG Windows Users Group, I came across an announcement of Microsoft iSCSI Target 3.3 is now available for download. Free!!!
All you need now is to point your iSCSI clients (HyperV and ahem others) to the Target and you are ready to roll.