Backup Permission Issues

Has there been changes to backup?
Previously I have set permissions to include NT Service\MSSQL$SQLEXPRESS and shared than mapped that folder to each terminal with the same drive letters so every machine can backup to same location on the DB hosting machine.
However am strugling.
Can backup to C:\TILLS\BACKUPS
Tills folder is shared and mapped to T:\ drive
But T:\BACKUPS I get SQL Server does not have permissions to access to the Backup Folder.

Forget it, needed to use \\EPOS-1\TILLS\BACKUPS type directory url