Make sure you have gone through the checklist mentioned in KB.
VMware KB: Unmounting a LUN or detaching a datastore/storage device from multiple VMware ESXi 5.x ho...
I've checked all of these before. It didn't help Is there any command on Power CLI to find out whether datastore related some object in vsphere environment or not? By the way I'm using vsphere 4.1
Does any of your VM's have snapshots? Maybe an ISO was mounted at the time the snapshot was created!? In this case you will need to delete the snasphot. You may also check your VMs "Summary" tabs in the vSphere Client, to see whether the datastore you want to remove still shows up for one of them.
I've checked snapshots too. What a dilemma ha?!
Can share a couple of screenshots.?
Could you post ESX shell output, please? Rescan your SAN again. What is the array you are connected to?
Last resort - you can destroy or unmask the LUN on array and recreate/reconnect it later.
What did you mean about ESX shell output? The SAN storage is IBM DS4700. I have tried to delete datastore by deleting mapping from storage appliance but it haven't worked ether.
I meant for example output of ls command issued from ESX host console. Did you try to delete the volume from different ESX? Try "esxcli system coredump file list" than check if coredump file is locking your volume
You can find detailed procedure here Cannot remove datastore * because file system is busy. | Virten.net
Have a look here http://www.vmadmin.co.uk/vmware/35-esxserver/90-vmfslockedfilerelease
Try to use vmkfstools with -B or -L switches
It's DONE! I don't know why but when I tried to get the error message again, it deleted without any error. All I did recently was installing VAAI driver to all 3 hosts. I didn't do anything else. Sebek, I think core dump files are used in version 5 and higher it would not help me anyway. But anyway THANK YOU GUYS for your RESPONDS! I appreciate it!!!