To import a VM from VMware Server to ESXi one must convert the disk by cloning the old one using vmkfstools. Fair enough. Doing this using the ESXi RCLI tools should look something like (based on what I've seen for non-RCLI for ESX):
vmkfstools [conn-options] -i old-disk new-disk -d thin
However, when you do this with RCLI, you get the following useless error message:
Unable to clone virtual disk : A general system error occurred: Internal error
Using the --verbose flag to look at the gory SOAP details I could see that the thin option wasn't getting sent. Looking through the Perl code it looks like one needs to specify both the -d and -a (adapter type) options. Once I added -a lsilogic it worked like a charm.
My big complaint (and I have other examples of this up my sleeve) is if the user makes a simple, bonehead parameter error, the command should point it out rather than saying "general system error...internal error." I hope this post will help someone else out if s/he is unlucky enough to encounter this error message.