Before an operating system can read from or write to a disk, the file system on one of the disk’s partitions must be mounted. A “mounted” disk is available to the operating system as a file system, for reading, writing, or both.
When mounting a disk, the operating system reads information about the file system from the disk’s partition table and assigns the disk a mount point.
Windows and macOS, by default, automatically mount every disk that is attached to the computer. For instance, when you insert a USB flash drive, the OS detects the new storage media, attempts to mount it, and if it can, it does.
Example of usage – “Microsoft Windows, mountable file systems are called volumes. Mounted volumes are listed in File Explorer in the section named “This PC”. Every mounted drive is assigned a unique letter.”