Stuck on Prechecking Structures

I have a roll-your-own Fusion drive on a 2008 iMac that won't finish booting (even in Single User mode). The Fusion drive passes Volume Structures, and each physical drive passes Partition Map, SMART, and Surface Scan. But the drive won't mount, and Volume Rebuild gets stuck forever (more than a day) on Prechecking Structures.

Is there anything else I can do to try to save this drive, or am I stuck with reformating and restoring?