Take this with a grain of salt, because I work for Iron Mountain (although not the division that sells or supports these products and services.)
If you support a company, you shouldn't do this.
- What if someone writes or deletes a file? What if they do one thing on one drive and another on the other drive?
- What if the controller goes bad or you get an electrical problem? Now you have two copies of corrupted data.
- What if the whole room goes up in flames? Will you still have a tape with the data?
If you really rarely access the data, and will know when you need it again, throw it on a tape and get it offsite, like you do (or should) with your other tapes. You need it, order it returned.
If you know that you rarely access it, but when it's needed, it's needed NOW, we have a product called VFS that lets you seamlessly migrate unused files to our storage and off your disks, but it still appears where it did before. That gets you the reduction in currently-used disk space (and thus reduction in backup window and used tape capacity), plus security for your data. Like I said, grain of salt, but it may fit your needs well.
Alternatively, you could set up a second cheaper server, with slow/cheap/non-redundant disk for cost savings. Put your old files there, and use DFS to present a unified file tree, and only back up the second server monthly (or whatever fits your business needs.)
Or you could just archive this junk to a bunch of DVDs, 2-3 copies each. Give one copy to the user(s), and put the rest somewhere secure, maybe on-site and off-site.
If this is just for you, most of this stuff still applies, but the cost/benefit can be significantly different. If it's just you, I'd lean towards archival DVDs.