When you detach a volume, it will be available in the virtual datacenter again. For ESXi, you can only detach an iSCSI external storage volume if the VM does not have VMware snapshots. If you cannot detach a volume as expected, please contact your System Administrator.
When you detach a hard disk, the platform will destroy it and all data on it.
To delete a hard disk or to detach a volume of virtual storage:
Go to Virtual datacenters → edit VM → Storage
Select the hard disk or volume
Click the detach/delete button at the top right of the Storage tab
Confirm the delete or detach operation
Click Cancel to discard changes or Save to reconfigure the VM in the hypervisor
A volume that has been detached will now be available again in the virtual datacenter.