VMWare ESXi 3.5 "Could not power on VM: No space left on device"
Solution 1:
How much memory does the VM you're trying to power on have?
ESX needs to create a swap file the same size of the VM's allocated memory when it's powered on; this swap file is usually create on the same datastore the VM resides in (but this behaviour can be changed).
So, if f.e. the VM has 4 GB virtual RAM, you'll need 4 GB free on the same datastore where the VM is.
If ESX is configured to create the swap file somewhere else, you'll of course need that much space there.