Computer loses Portable Passport Drives

I have 7 Pass ports on my computer; I use them like 1 and 2 TB thumbdrives. I have been having the same problem with some of them periodically. This time it was 2 of them, and I found that going in an re-assigning the same Drive letter to them, fixed the problem at least for now.
I want to know why this keeps happening. I do not have all of the drives turned on at the same time. Sometimes the computer will tell me there is a hardware problem with the dirive, but connected to the laptop, it works fine. All of these problems are why I have so many duplicate copies of my files.