[CentOS] Possible to use multiple disk to bypass I/O wait?

I'm trying to resolve an I/O problem on a CentOS 5.6 server. The process basically scans through Maildirs, checking for space usage and quota. Because there are hundred odd user folders and several 10s of thousands of small files, this sends the I/O wait % way high. The server hits a very high load level and stops responding to other requests until the crawl is done.

I am wondering if I add another disk and symlink the sub-directories to that, would that free up the server to respond to other requests despite the wait on that disk?

Alternatively, if I mdraid mirror the existing disk, would md be smart enough to read using the other disk while the first's tied up with the first process?