I made this mistake once, on accident. I had dragged and dropped about a thousand ebooks/articles/etc onto a Kindle I had connected to my PC. I thought I had dropped the payload onto my other external drive.
Later, after I realized that my Kindle was moving slower than tar coming out of a bucket in a Maine January, I saw that I had 1100~ books somehow on my Kindle. And, none on the external drive. Thankfully, the freakout was averted.