vmkfstools -i bitnami-owncloud-4.5.5-0-ubuntu-12.04.vmdk owncloud.vmdk
On my ESXi 5.1 box this command promptly failed with a very confusing error message:
"Failed to open 'bitnami-owncloud-4.5.5-0-ubuntu-12.04.vmdk': The system cannot find the file specified (25)."
Of course the file existed in the current directory, so how could it not be found?!?
After doing some research I finally was able to understand and fix this issue - it is described in VMware KB2036572: In ESXi 5.1 the VMkernel multiextent module is no longer loaded by default, and this module is needed to support virtual disks of hosted formats (like this one). Later I also found a blog post by William Lam about this issue (I must have missed that ...).
The solution is easy: Manually load the multiextent module by running
prior to the clone command. This worked as expected for me. After cloning the disk you can unload the module again with vmkload_mod -u multiextent.
This is something to keep in mind ...