This is what worked for me :)
I had the same issue, and could not find the fix anywhere. I tried every fix I found in any posts. But nothing worked. I could not format or mount my SD Card.
Then, I noticed the "internal sd storage" said "not available"...
... I thought hmmmmmm... so I formatted it.
And after that, I could format the External SD Card and mount it...
Hope this helps somebody else.
Apparently, if it fails to mount the internal storage, it will fail to mount the external as well...