My /Users folder is on the boot drive.
On another drive, I have a folder (F) that I want to protect by providing no access to other users.
How can that be done without encrypting F?
You can't unless you are willing to accept some serious limitations.
The operating system will enforce ACL and normal posix permissions, but that presumes no-one will have an administrator account and no-one will access that external drive with any other OS than the one you have set up with the permissions.
If you're OK with this light level of security, just use Finder and the Get Info pane (or the inspector) to set no access to folder F for anyone but your user.
Here is the basic (dry) article on how permissions work, then a more wordy tutorial on ACL that let you say no one can read a folder and lastly a reminder how easy it is for someone else to change those permissions by using an admin account on the OS you set up (or if they connect it to another computer that they control)