Why is my file system mounted as read-only?

Try to search in dmesg | less.

If you would like remount it to read-write, use mount -o remount,rw /

I had a similar issue that is not solved with the solution above. Linux may mount a drive as 'read-only' because of Fastboot on Windows 10. A shutdown with the Fastboot feature keeps the Windows kernel and session running albeit it closes all applications and logs off users. By turning off Fastboot, windows will do a cold shutdown and linux will be able to mount the drive safely. You will loose the fast windows boot as a consequence.

To turn-off fast-boot on windows 10 do the following:

1. Go to Control Panel.
2. Click Power Options.
3. Choose "what the power buttons do."
4. Click "Change settings that are currently unavailable."
5. Find and uncheck "Turn on fast startup."
6. Save changes.
7. Shutdown and boot linux.

References: https://itsfoss.com/solve-ntfs-mount-problem-ubuntu-windows-8-dual-boot/ https://www.howtogeek.com/243901/the-pros-and-cons-of-windows-10s-fast-startup-mode/