Scenario
VMWare ESXi is being used in conjunction with external storage (IBM SAN). We needed to upload VMWare images and virtual disks from one ESXi server to the external storage. Then, we needed to “mount” the same storage without losing the pre-existing data.
Maybe counter-intuitively, using the “add storage” option in the vmware configuration page will actually format the storage, meaning all data is lost (configuration > storage > add storage)
The trick is simply ignoring that option, and mapping / mounting the storage to the ESXI server only within the external storage itself. For example, with the IBM storewise the process was:
1. On the SAN, map the volume to the first ESXi server
2. On the ESXi server, under configuration > storage, hit “rescan all” and “add storage” to add the newly mapped volume as a VMFS disk
3. Perform any needed operations, eg storing the VM images on the newly created volume, etc
4. On the SAN, unmap the volume from the ESXi server. Re-map the volume to the new appropriate ESXi server
5. On the new ESXi server, do not hit the “add storage” button. Simply wait for about 5-10 mins, and hit “refresh” a number of times.
The ESXi server will recognise that a VMFS partition already exists on the disk and will display it as an available volume, retaining ay existing data.
David Vassallo's Blog 