how do i double check that my flash drive backed up everything on my computer?

[lalala]

I am using a My Book Essential that holds 1.7 TB on my macbook. Afew times while backing up my memory, it said that the device was removed and data may have been deleted this was because the power connection was being very sensitive, but anyhow, before this happened everything was going fine and the transferring of the data was taking its time backing up the information, but after the device was unplugged by accident, it says its backed up, but I don't believe it since it didn't take half the time it was taking to do only a little of the memory on my laptop