gdu | Fast disk usage analyzer with console interface | Command Line Interface library

The error you're getting indicates that you're trying to write on a file system that's read only. If you look at the mount output for /, then the ro mount option indicates that it's read only.

There are usually a couple reasons that your file system is read only, but in this case, it's likely the behavior of an ext4 file system when it encounters a file system error. The default is to remount the file system read only to avoid further damage but allow the system to continue. The alternatives in this case are to ignore the error and panic, both of which are generally less desirable.

You can verify this by running dmesg in order to see whether the kernel has reported any file system errors and remounted the file system. If so, you can generally reboot the system and an fsck will occur automatically. You may need to boot from a different medium (such as a live CD) to perform the fsck (by running e2fsck -f /dev/sda2) in some cases, since not all errors can be fixed when the file system is mounted.