signal light of USB thumb drive

[mangada]

Some USB thumb drive has signal light, which will flash when the USB thumb drive is being accessed, is my mind right?

[sudodus]

What do you mean by 'right in most cases'?

I think it flashes when read from or written to.

But generally, both for USB drives with and without signal light, all partitions on it should be unmounted before you unplug it. Run

sudo umount /dev/sdx*

where x is the drive letter (for example a or b) in a terminal window and wait for the prompt to appear.

You can also click on the eject symbol if you use a graphical tool (for example Files) and wait for the eject symbol to disappear.

This way you can be sure that all write operations have finished (that all buffers are flushed).

[mangada]

During boot, signal light of USB thumb drive flashes more than 5 second?

[TheFu]

Some USB thumb drive has signal light, which will flash when the USB thumb drive is being accessed, is my mind right?

I'm not a qualified psychologist, so I can't say anything about your mind, but some USB flash storage does have an LED that flashes when active. I have a few like that, but most don't bother. It is a more expensive to manufacture flash storage with an LED than not to do that.

Under linux, as sudodus points out, we can't trust any LED on flash storage to provide a clue as to when it is safe to remove the device. It cannot be mounted or file system corruption is likely. The solution is to use the "eject" button in a GUI program or the umount or fusermount -u commands to un-mount the file system. Which command should be used depends on how the file system was mounted.