Should I use the DOMAIN\USERNAME instead of PC NAME to validated?
That depends on the PC and or domain, but it is quite simple to be sure. Just right click on the shared folder and select Properties
The Network path is the location, you can see mine is \\Hostname\folder name. So a template would only need the share listed as html.
Then select Advanced sharing > Permissions and add the username the MFP will use for scanning, then format it exactly as it is seen within the brackets, which you will notice is not the way you likely typed it when you added the user.
Does it need that the shared folder is in C: Partition?
As above is shown, you do not. I will note her that you can't really go by hostname, you need to scan to the PC IP address, the Connectkey machines don't have Windows Networking built in, so unless the device is registered on a Domain, it will fail with a hostname. If scanning to Server 2008 or higher, or Windows 8-10, you need fairly new firmware, luckily there is a tool installer that will run from your desktop, upgrade the MFP and no data loss at all occurs. At the latest firmware there is at this time, nothing you cannot scan to smb to as long as the settings are correct.
