System Monitor can monitor the current usage of storage space on disk volumes (file systems, partitions) currently connected to your computer. The total amount of storage space used by selected volumes is displayed in the menu bar in this case.
If you like to monitor used storage space of the system, perform the following steps:
Only those disk volumes will become part of the statistics which you have selected for monitoring (see below).
The data shown is calculated live and updated about once per minute. The disks will not be checked more often in order not to thwart transfers of actual user data. In addition, System Monitor respects your personal energy saver settings for hard drives. When the program has to assume that drives are currently in sleep mode, it will automatically avoid to retrieve usage data for volumes, in order not to wake up the drives. If you open this menu purposefully, however, all affected disks will be woken up in order to display up-to-date information.
In case you are using at least version 10.13 of macOS, System Monitor is capable of working with different definitions for free and used storage space. You can switch between these different display modes using the pop-up button at Definition of “free”.
The definition of “free” that you choose here becomes effective for all volumes shown by System Monitor in the menu bar and menu items. Note that the values for used space will change accordingly because the true amount of storage will never change of course.
If you enable the status bar in modern versions of the macOS Finder, the amount of free space shown in these bars of Finder windows will match the definition space after purge operation.
The box Menu Bar controls in which form storage space should be displayed. System Monitor offers a variety of icons and functions that you can combine in any order. The following elements can be used:
Within the box, the upper bar simulates as a sample how this monitor section should be presented in the menu bar. You can grab the individual items by mouse and move them into a different order. You can remove an element by pulling it out of the bar. The lower bar contains the stock of items available. If you like to add a certain item to the menu bar, use the mouse to drag it from the lower to the upper bar. Pressing the button Default causes your current settings to be deleted, replacing it by a default suggestion for the menu bar.
Spacers are shown with dotted rectangles to make it easier for you to recognize and to move them. In the actual display, the rectangle will become invisible, so the spacer does its job providing a blank area within the menu bar.
If you mistakenly have removed all items of this monitoring section from the menu bar, a three-star-icon (⁂, asterism) will appear as a placeholder. This ensures that you still can open the associated menu.
You cannot drag items directly into the real menu bar.
At least the following items will be shown for each selected volume as part of the menu associated with this monitor section:
The table Include the following volumes allows you to control which file systems should be monitored by the app and which shouldn’t. Set a check mark next to each volume which should be monitored and remove the check marks for volumes where no monitoring should be performed. The selection can also be applied to all entries automatically, using the buttons All or None. The lines in the table can be dragged to a different sort order if desired. This order will then be used for the corresponding sections in the menu.
If volumes uses the modern APFS (Apple File System) instead of the classic HFS+ (Mac OS Extended), this will be indicated by an additional marker APFS at the right side of the table. APFS supports advanced features where several volumes can share each others storage space. This may affect your choice which volumes you would like to monitor.
External and ejectable volumes will also appear in the table automatically. If you like to monitor a disk which is not always connected to your computer, this will be possible. Perform the following steps:
If this volume is being unmounted later, it will be shown using gray print in the table and it will keep its position. The monitoring will automatically continue as soon as the volume becomes available again.
In order to make sure that neither you nor System Monitor confuse disk volumes, you should take care that all volumes used have different names.