What does the following command do?
startup mount **exclusive**;
Deprecated .. now use STARTUP MOUNT
http://psoug.org/reference/start_stop.html
If your Oracle server allows multiple instances to access a single database concurrently (Oracle Parallel Server option), choose whether to mount the database exclusively or in parallel.
If you specify EXCLUSIVE (the default), then the database can only be mounted and opened by the current instance.
Taken from: http://download.oracle.com/docs/cd/A87860_01/doc/server.817/a76956/start.htm#6155