Apple - Using Terminal, how can I find which directory is my USB drive mounted in?

All drives (internal, external and networked) get mounted in /Volumes. You should see a folder there with the name of your USB drive as it appears on your desktop or in the Finder.

Some techniques to try:

ls -a /Volumes

or...

ls -l /Volumes

or...

ls -la /Volumes

...may prove useful.

mount

seems to give all the partitions that might be useful with Disk Utility, during formatting, etc.

you can probably reach your USB drive directly using:

/Volumes/<NameOfYourDrive>

This applies to other types of drives as well: optical, internal and external hard drives. All mounted volumes.

In general, to find where something is mounted, you can run this command from the terminal:

df -H 

This also provides useful information about the space on the drive.