Sometimes it helps if you put one "dummy" picture on a drive via Win Explorer. So, try this first and see if your drive shows up.
If this doesn't help:
Letter B is not a good choice for a drive on a PC. This is because of ol-old times when drive letters A and B were reserved for the 6.25" disk (A) and floppy drive (B).
That's why the first drive in today's PCs is still named (lettered) C.
And, I don't know maybe Lr does not recognize drive letters A and B. So rename your drive B to drive D.
You do this in Windows Control Panel >System and Security >Administrative Tools >Computer managaement >Disk Management. In the lower window where it shows your drives, highlight the drive in question then go >Action >All Tasks >Change Drive Letter and Path.
For more info open <Help> in your Control Panel and type <drive letter> into the search box.