When scanning you might see a message Could not scan /… or some of the folders it contains at the top of Disk Usage Analyzer window. This error appears because you don't have the required permissions to access some files due to restrictions set on the target system. The files you can't access won't be used to compute the chart representing the disk usage, hence the result reported may be wrong.
Not having access to all files and directories is perfectly common though, so there is nothing you can do about this error.
Disk Usage Analyzer uses command du to create the detailed view of the use of storage, and needs you have read permission on files and execute permission of directories.
On the other hand, filesystem disk usage on the use command df for disk utilisation on the main window which relies on the filesystem and not on file permissions.
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