Your hard drive is being assigned a different letter than it was before. If before your hard drive was letter D, if you plugged in a flash drive first and then the hard drive, your hard drive might now be letter E. Easier would be to plug things in order so that the drive ends up with the same letter every time.
What I often do with external hard drives that I use often, I assign them a letter that is very high, like P, for example. That way, if I plug in any drive temporarily, it won't affect the location of what I have on those external hard drives I use often.