I have another question.
if I unplug the USB while the Xubuntu computer is off, ie, power off,
is it safe?
Yes, you should be okay... at least most of the time. The reason you want to safely remove the drive is that Linux handles writing data to a USB differently than Windows. With Windows, if you copy a file to USB, when the copy is finished you can just yank the USB out of the computer... Windows was really finished. Linux often does
NOT really finish... when you tell it to copy a file to USB, it acts like it is done... but actually a lot of the data may still be holding in RAM waiting to finish. So, when you tell it to safely remove the drive, then the RAM is flushed and the copy completes before you remove the drive. Otherwise, if you remove it too soon, the copy of data may be incomplete.
This is one of the very, very few things that I think Windows does better. But Linux has behaved this way forever, so do not expect it to change anytime soon. You just have to remember to safely remove the drive before you physically unplug it from the computer.
If you were copying a lot of files to a USB, and then powered off the computer before the copy was complete, you might still have a problem like described above. So it is still better to safely remove the drive before powering off. But once you have powered off... it doesn't matter much at that point, so you might as well unplug the drive. But you might consider taking a look to make sure it is okay when you boot the computer next time.
Cheers