Unable to create a tablespace, unusual error
Tracked it down. I set FILESYSTEMIO_OPTIONS=SETALL the other day. It appears that direct I/O works for the local RAID array but not for the external. When I set it back to NONE, I was able to create tablespaces on the external device. 

A follow up: Does anyone know if you can enable direct I/O on a per device basis without using ASM or RAW?