Our LSI 9650SE-12 RAID Controller dropped the main Postgres disk offline ... it just disappeared as though the disk wasn't there. It was an 8-disk RAID10 unit. The other unit (RAID1 for Linux & pg_xlog) was still functional.
Using tw_cli, it showed the array as "DEGRADED" and claimed to be verifying it. One disk in the array was "DEGRADED". There was no /dev entry for the device; Linux couldn't see it at all.
Aha. I found this. Check out the first item in the "bugs" section: "RAID-10 arrays going Inoperable/Verifying Mode (SCR-2278)".