Harddisk becomes Read-Only

I have QNX4.25 installed in a Compaq ML370 with 2 harddisks (9.1GB each)
attached to AHA 3940 SCSI controller. I have been running the same program
for almost one year. Recently when I shutdown the system and reboot it, one
of the disk (the one bootable) has become Read-Only. None of the files in
the disk is writable. Has anyone experienced this? Any suggestion what might
went wrong? Thanks.