It's looking like one of the little drives (hde) is starting to fail. I'm running a bad-block scan to make sure hdc is in good working condition; if it is I'll pull hde and do some diagnostics on it. It's already been pulled out of the RAID, so it'll need a rebuild anyway. Even if I have to replace it, my brother is due for a hard drive upgrade; I'll take his old one when he gets the new one.
Navi-chan's back online. I pulled hde out, so she's now running on hda and hdc only, but that shouldn't affect much.