“/etc/mtab is not writable”, “read-only file system”, Linux, Virtual Machine Problem Solution

if you are experiencing problems with Linux or Linux Virtual Machines as:

"read-only file system"

on file operations or

 "/etc/mtab is not writable"

on return of typing command “mount”.

Errors are like this:

mount: warning: /etc/mtab is not writable (e.g. read-only filesystem).      

            It's possible that information reported by mount(8) is not      

            up to date. For actual information about system mount points      

            check the /proc/mounts file.

Solution:
Run fsck to fix the device disk, then reboot afterwards

fsck  /dev/sda1