I would like to know the standard procedure to unregister the archives from the SMT and shift this back to shared folder to make my backup drive free space for further daily pi backup
To unregister, see this link.
After unregistering, you can go to the folder where your archives are and just cut and paste them into your shared folder on another drive to free up space.
A PI Data Archive backup standard method of taking backup of your data and configuration files that can be used to restore the system in the event of an data corruption, unintended configuration change, or a hardware failure. There are different types of Backup available, refer the below KB article which contains recommended procedure to backup the PI data Archive,
If you are looking for automizing your daily pi administrator work, then you should really get the Powershell Tools for PI System. This is usually included with PI-SMT.
Sample scripts are within your %PIHOME%\OSIsoft.Powershell\ExampleScripts\MoveOldArchives.ps1
Its syntax can be found here: PowerShell Tools for the PI System
In addition to your question, you no need to unregister the Archives in the backup drive since it was not registered. Archives available in Archive drive only registered archives.
So you can directly move the archives available in backup drive to the external drive.
Retrieving data ...