For those of you still doing manual backups, please note the following. When a computer writes to a USB key or any other drive what the computer does is write to the cache which is its memory. Next it reports to itself that it has done this successfully and only then it writes to the USB key. This is what is being reported when you can see a light flash on and off.
Sometimes I have heard of it taking up to ten minutes from when the computer starts the backup, and the lights stop flashing. During this period, the computer can appear to be frozen. If you try to do anything during this time, there is a significant chance that the system will overload and lock up. So do not do anything with this computer.
On a network, this means that all machines maybe knocked out. There is nothing anyone can do but wait until the green light on the front appears. As soon as the light goes out, the system will have finished writing.
Do not like one has done today pull out the USB key early.