When you insert the USB it becomes a drive. It also has a name, you can read the name of the USB so depending on how many you have plugged in you can work out the drive designation.
Command: (vlax-dump-object drv)
; IDrive: Drive Interface
; Property values:
; AvailableSpace (RO) = 1.20117e+11
; DriveLetter (RO) = "F"
; DriveType (RO) = 2
; FileSystem (RO) = "FAT32"
; FreeSpace (RO) = 1.20117e+11
; IsReady (RO) = -1
; Path (RO) = "F:"
; RootFolder (RO) = #<VLA-OBJECT IFolder 000001bf486ea458>
; ShareName (RO) = ""
; TotalSize (RO) = 6.3995e+11
; VolumeName = "SAMSUNG"
Your usb should have a name, is this what your after working out which one to write to.
https://www.cadtutor.net/forum/topic/9848-how-to-get-serial-number-of-hard-drive-by-lisp/ code by ASmi replace 'serialnumber with 'Volumename