I suggest adding the following to the Developer's Guide after "Double-booting system behavior":
Booting a database in read-only mode
If Derby is not able to create the db.lck file when booting a database, the database will be booted in read-only mode. This may happen due to lack of disk space or access rights for the database directory. In this case, the boot message in the error log will look like the following:
2005-10-04 12:12:30.835 GMT:
Booting Derby version The Apache Software Foundation - Apache Derby - 10.2.0.0 alpha - (293288M): instance c013800d-0106-bb8c-12e6-00001672f894
on database directory sample in READ ONLY mode
See also "Creating Derby databases for read-only use"