Linux – File system keeps becoming read-only

When that happens to me with a fixed disk, it means the disk is dying. Most likely this is what is happening here. If this is a backup drive that is repeatedly connected/disconnected/transported-between-locations, it is very possible that a shock or repeated thermal changes have resulted in a flaw. Most of these USB drives are not specially protected against drop/shock or thermal changes, they are just a standard SATA drive in a USB-to-SATA plastic housing.

My rule of thumb for disks, especially when it comes to backups, is: if there's a doubt, throw it out.

To rule out the USB infrastructure, you could run the disk extensively on another computer, which doesn't actually solve your problem since you still have to back up the computer.

The filesystem is probably mounted with the option errors=remount-ro, which as the name suggests means that if an error is detected, the filesystem is immediately set to read-only, to avoid further damage.

There will be information in the kernel logs (/var/log/kern.log on most Linux distributions).

What to do next depends on the cause. Here are the most likely:

• It could be a failing disk. Often you'll see IO errors reported in the kernel logs. smartctl -a /dev/sdb can tell you more. Back up your data as soon as possible and replace the disk.

• It could be a problem with your RAM. Run a memtest just to make sure.

• It could be a kernel bug. This is hard for mere mortals to diagnose. Make sure you have the latest kernel released for your distribution.

• The filesystem could have been damaged earlier, for a reason that no longer applies (e.g. a kernel bug that has now been fixed). Running fsck should fix the problem, so unfortunately for you this case doesn't apply to you.