It is difficult to know what is happening.
You could save a copy of the home and opt directories, do what I suggested in the previous post, and compare the old with the new.
There are many possibilities. If you are running from a usb drive, maybe the usb drive is faulty and it will happen again.
Maybe you have done something unintentionally to cause this. Is that a possibility?
Did you use USB Install or another method? If you used another method, you may have done something wrong.
Has it only happened once, or often? Have you had backup working correctly at other times?
Because there is more than one error, I suggest you start with the basics, such as determining whether the usb drive is faulty or not. If you find out it is faulty, you need another drive. If you find out it is not, you can then consider other possibilities.
If the usb drive is faulty, maybe other files are corrupted or lost.
Doing what I suggested in the first post will determine if it is easily fixable to begin with.
If it is fixable, using it will determine if it happens again.
If you get another error during backup, see if you can get more information next time.
If you find it is not easily fixable, and files are corrupted or lost, reinstall Tiny Core (using USB Install) and see if it happens again. If you do everything correctly, and you continue to have faults, it is very likely that the usb drive is faulty.