Each device listed under the File Systems tab is a storage disk (like a hard drive or USB memory stick), or a disk partition. For each device, you can see what its total capacity is, how much of its capacity has been used, and some technical information about what type of filesystem it is and where it is "mounted".
The disk space on a single physical hard disk can be split up into multiple chunks, called partitions, each of which can be used as if it were a separate disk. If your hard disk has been partitioned (perhaps by you or the computer manufacturer), each partition will be listed separately in the File Systems list.
You can manage disks and partitions and see more detailed disk information with the Disks application.
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