Hi Leonardo. I'm Greg, a volunteer installation specialist and 8 year Windows MVP, here to help you.
Type CMD in Start Search, right click Command Prompt to Run as Admin, paste these Commands each followed by Enter:
DISKPART
LIST DISK
SEL DIS # (replace # with the external drive number)
LIST PART
SEL PART # (replace # with the external's partition number)
ASSIGN
EXIT
If that fails install free Partition Wizard, right click external to Assign a Drive Letter. http://www.partitionwizard.com/download.html
If that fails and drive is empty, delete it's partition in PW, run Surface Scan to find it's condition, then Create a New NTFS Logical Partition, assign a letter. https://www.partitionwizard.com/partitionmagic/...
I hope this helps. Feel free to ask back any questions and let us know how it goes. I will keep working with you until it's resolved.