Turns out you don't need to switch the USB port to avoid the error (at least with this laptop model); you simply pull out the thumb drive after exit diskpart; and insert back into same port. After which, MakeWinPEMedia cmd doesn't fail!
Great tutorial!
In Step 3 I ran into an issue with the MakeWinPEMedia CMD, it kept throwing error:
DiskPart errorlevel -2147212269
Of course googling didn't turn up any results with that exact error (different error level codes), BUT I found the solution - the particular USB port on this Dell...