Tiger will soon release poolIt for Mac, a new software tool that allows you to create a virtual storage pool by dynamically adding (or removing) live HFS+ volumes into (from) it.
The pooled volumes are hidden and replaced by a single mount point. When exploring the pool with Finder (or other software), the content of all pooled drives appear as "merged" (just like if you had copied multiple these drives onto a large one). The available free space is the total found. poolIt is safe and non destructive. There is no reformatting of your underlying volume. poolIt is a virtual file system that simply provides a different view. This is why you can safely add or remove volumes from the pool without affecting your data.
Tiger is now looking for a handful of testers. The technology is stable and risk free. However, we want to confirm it works with all the software you are using. If you are interested in taking a look at poolIt, just drop me a note prior to Sept 15th at bernard at tiger-technology.com. poolIt can be used with a combination of all types of drives (SAS, USB, FireWire, etc.), as long as they are formatted using HFS+.