When doing things the normal way, Windows assigns the next available drive letter whenever you plug in a USB drive (although 'A' to 'D' are normally reserved for two floppy drives, even though they're no longer used these days, the main hard drive (or SSD) and an optical drive). So, if you insert a USB drive, when there are no others already present, it will be assigned 'E' as its drive letter. If you then plug in a further USB drive, it will be called 'F'. If, when you next use your computer, you insert those drives in the opposite order, your old 'E' drive will now be you new 'F' one (and vice versa).
To get around that problem, you need to assign a persistent drive letter to each device. See here:
https://www.howtogeek.com/96298/assign-a-static-drive-letter-to-a-usb-drive-in-windows-7/
(Don't be put off by the reference to Windows 7 in that URL. The instructions apply to later versions of Windows too).