Question : VMware where to map the datastore?

I am following this method to add a hard disk to a virtual machine from a  LUN (Lefthad iSCSI SAN).
http://www.vmadmin.co.uk/vmware/35-esxserver/58-rdmvm

At the step where you select a datastore:
What is the difference/benifits/disadvantages of mapping the datastore onto:

The Virtual Machine?
Secifiy data store (Which would be the Main Datastore, Host hard drive)?

Answer : VMware where to map the datastore?

I would suggest:
Remove the GC role from the server/s that are not functioning.
Reboot the server & force replication.
After you are sure that replication has completed, re-add the role.
Again, force replication, then run DCdiag in verbose mode & check for errors.