To find the volumes and volume locations in PostgreSQL the steps provided in Finding PostgreSQL Storage Locations can be used.
Microsoft Windows - Press the Win + R keys to open Run, type diskmgmt.msc into Run, and click/tap on OK to open Disk Management. Right click or press and hold on the drive (ex: "PG_Base") you want to unmount, and click/tap on Change Drive Letter and Paths
Select the mount point and click on the Remove button and then the OK button.
Linux - In a terminal execute the umount command:
umount /var/lib/pgsql