The easiest fix is to use the disk management tools to reassign the external drive the letter desired.
My recommendation is to pick a letter above "J" so that any SD drives or the like do not keep changing the letter on you.
After you have the letter assigned, go into Lightroom's Folder panel.
If the folder tree is not shown like Explorer does it, right-click on a folder and select "show parent". Repeat as needed.
Then "relocate" the top-most folder to the drive with the new letter. Everything under that top-most folder will be automatically updated.
Let me add that IF you have that external connected "Before" you connect a SD card the drive letter will never change. So always have the external connected, and awake, before you insert/connect a memory card.
Personally I assign letters to externals starting with Z and working backward. That way no drive I connect, internal, external or memory card, will get the drive letter from any of my external drives whether they are connected or not.
ManiacJoe - Thank you very much... it worked for me
Good idea... thank you very much