Strange errors like that are one of the reasons we're trying to step away from the Flash plugin, but that's one I haven't seen before.
Can you describe a little more about this VMDK attaching issue? It sounds like:
1) You created a VM
2) You want to add multiple harddisks on it using the new HTML5 based vSphere Client for vCenter (or are you speaking about the Host Client for managing single hosts?)
3) You do so by going to the VM and clicking "Edit Settings" and adding new disk
4) When adding the new disk, you specify that it should be Thin provisioned, and the Location should be on another Datastore (not "Store with the virtual machine").
Please correct me if my understanding is wrong.
Questions:
a) What versions are you on: Host, vCenter if applicable?
b) Did this flow work differently on the Desktop client?
c) I didn't quite understand the situation with your datastore's free space and your Thin/Thick decisions. It sounded like your ds1 is a 1TB disk with 70 GB remaining, and you want to create additional VM harddisks which are 6 TBs? I'm sure I missed something here.
d) What prevents you from specifying Thick provisioning when creating the VM: does the UI report an error? What error?
e) When you do change the location (in Step 4), does this new location fail to hold, and the VMDK actually ends up in the same location as the root disk?