Mongodb error on starting

First of all, look for other instances of mongod running on the system using the age-old command:

ps ax | grep mongod

If you see a line like -

98555   ??  S      4:40.89 mongod --dbpath /Volumes/ComputerName/data/db 
-PID-                      -name-          -------------path------------

then there is a mongod process running already. If so kill it and then start mongod again.

To kill a process with PID = 0000, in Unix like environment use -

kill -9 0000

If your console returns something like, -bash: kill: (98555) - Operation not permitted, use

sudo !! 

to repeat the command as super user. This should get you going.

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PS: If you haven't stored any data in your db yet, delete /data and then create the /data/db directory again. Do a sudo mongod and it should work.