Hi

To resolve this issue, you must locate the virtual machine that has this .vmdk mounted and then unmount it. To do so Search for references to the .vmdk file in the settings files (.vmx files) of the other virtual machine's on the datastore. You can automate the search with a command like this

    find /vmfs/volumes/* -name *.vmx -exec grep -Hi <name-of-the-file.vmdk> {} \;

    where <name-of-the-file.vmdk> is the .vmdk you're trying to delete.

After you have located the virtual machine which has mounted the .vmdk, remove it. Select that virtual machine and go to Edit Settings > Hard Disk > Remove.