@pyrre
But that's the problem, isn't it? How do you effectively manage the available storage on those four 1.5TB disks? If you use something like GlusterFS, each disk's storage can be managed as an independent file system, and the cluster file system will make them appear as a single logical pool. GlusterFS requires a client, but you can use a client box to re-export the data as CIFS, NFS (barf), or whatever.
Personally, I would just eat the cost and create a simple disk array, probably RAID5. Keeping tack of smaller drivers is a serious pain in the ass, and I use virtualization rather than real hardware to explore new x86/x64 software. Right now, I have two 1TB disks in a mirror. Everything else is on a shelf. (The Amigas do their own thing, however, mostly with old and small 2.5" disks.)