I have added the admin host IP address in the filer butstill I am not able to access the C$ of the filer from the admin host. I sawsome document saying that Root volume security style has to be NTFS and filer must be licensed with CIFS. If we can access the C$ from windows it will be very helpful for upgrading the firmware .
You are indeed correct about the usefulness of accessing C$ for upgrading ONTAP or other FW.
I have run into a few issues accessing C$ in the field before so hopefully can help.
Can you tell us what model the controller is, what version of ONTAP you are running? if you have a CIFS license or not? if you have have you joined to the domain? ANd what security style you have set on vol0?
If you have a CIFS lic and a free network interface it can be handy to setup CIFS in a workgroup and direct connect a laptop to the free port with each device having a fixed IP