When you are trying to unmount/delete a datastore you can get on of the following error messages:

Cannot unmount volume <datastore> because file System is busy

or

Cannot delete volume <datastore> because file System is busy

 

To resolve this issue proceed as follows:

 

If you are not using VSAN:

1. Connect to your ESXi Host via SSH

This Service isn´t enabled by default. You can enable it

a. with the vSphere Client: in the Security Profile in the Configuration tab of your ESXi Host

b with the Web Client: in Manage -> Settings -> Security Profile of your ESXi Host

2. Stop the vsantraces by running the command:

#/etc/init.d/vsantraced stop

3. Perfom a Refresh for Storage

4. Unmount/Delete the datastore.

5. Start the Service by entering the following command:

#/etc/init.d/vsantraced start

 

To permanently turn off this Service run the following command:

chkconfig vsantraced off

 

If you are using VSAN:

1. Run this command to change the Location of the VSAN trace:

# esxcli vsan trace set -p <datastore>

2. Unmount/Delete the datastore

 

Source and further information: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2069171