On a sunny day (Sun, 5 Aug 2018 11:15:24 +0000 (UTC)) it happened Martin
Gregorie  wrote in :

>On Sun, 05 Aug 2018 08:25:34 +0000, Jan Panteltje wrote:
>
>> Yes, I have the old Unix habit of typing 'sync' 3 times every now and
>> then.
>>
>> An other thing I notice with my USB sticks is that even after copy and
>> sync the LED on the USB stick keeps flashing, sometimes for up to a
>> minute or so.
>> I once asked in a Linux group if it was safe to remove the USB stick
>> when that LED was still flashing and was told that was OK...
>> Nevertheless I always wait until it is off.
>> For SDcards it is hard to tell, no LED, but I think the internal
>> controller on the cards / sticks does some housekeeping even after the
>> copy has finished.
>> So maybe it is better to leave it in for a minute or so longer.
>>
>Its more important to *always* unmount the card before removing it, i.e.
>click 'Unmount' on the card's icon if you're using a graphical file
>manager or use the "umount"  command if you're working from the command
>line.

Indeed, of course only when it was mounted first :-)
If you just copied _from_ an image umount is not needed.

dd if=/dev/sd* of=myimage.iso
does not need a mountg and umount.

>The unmount process waits for any activities using the device to end,
>makes sure any cached data is flushed to the device and disconnects it
>when all related activities have ended. Do this for all removable storage
>media, not just SD cards and USB sticks.
>
>NOTE that 'sync' just flushes cached data while 'umount' does quite a lot
>more. 'umount' will not allow a device to be unmounted if:
>- there are still files open on it
>- a process's working directory is on it
>- it contains a swap file that's in use.

Yes, the interesting thing is that even after umount the USB stick LED will
flash for a while.
Try it.

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