But I don't have new ones. Read priors at https://www.google.com/search?client=opera&q=can%27t+change+drive+letter+q
I had to reinstall Win 8.1 last night. My drive configuration hasn't changed in any way, but the drive letters assigned this time were different from the previous ones. I was able to change all my drives to their original letters except for one.
Between my two physical drives, I have a lot of partitions - 14 (don't ask) - of which one was drive Q. Now, Q isn't an available letter, even though no other drive is using that letter.