Anyone can help me? I am using Samsung SSD external disk installed Ubuntu 18.04. It appears this kind of problem when I keep it aside for a while.
Basically, the power saving features of your drive are triggering a bug that leads to the failures you're seeing - disk io errors.
Typical symptoms are the logs you show above, fonts not rendering, and your desktop environment crashing. While the issue has been identified for some SSDs, it is still a problem for some of us. I am also experiencing this issue.
As a workaround, You can prevent this by adding this to your start script in grub:
Although, others report that only by setting the value to
0 will you stop running into the error. Of course, then your SSD's power performance suffers.
The real bug
This is a bug that is partially reported here: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1805816
If you want to help fix the real problem, add more details about your problem in your initial question here in SO and in the bug tracker I link to above. For example, Look at the person who opened the ticket to see what relevant info he included. I'm just starting to tackle this myself by adding my details.