This procedure is different from moving disk volumes from one storage pool to another. See VM Storage Migration
A volume can be detached from a guest VM and attached to another guest. Both CloudStack administrators and users can detach volumes from VMs and move them to other VMs.
If the two VMs are in different clusters, and the volume is large, it may take several minutes for the volume to be moved to the new VM.
If the destination VM is running in the OVM hypervisor, the VM must be stopped before a new volume can be attached to it.
Log in to the CloudStack UI as a user or admin.
In the left navigation bar, click Storage, and choose Volumes in Select View. Alternatively, if you know which VM the volume is attached to, you can click Instances, click the VM name, and click View Volumes.
Click the name of the volume you want to detach, then click the Detach Disk button