Hi,

I have had this situation two or three times. In every case I saw from
ADSM's activity log a write failure to the volume immediately before ADSM
changed the volumes access mode to READONLY (which is a result of the write
error).
The write errors I assume are results of failing drive cleanings. I have a
Magstar 3575 L12 an a 3570 B02 and since autocleaning is on I never had
aproblem like yours.

Hope this will help.
regards
Rainer