Q Is there a way to lock an individual folder, so that it can only be opened on entry of a pre-set password?
A The options for doing this are surprisingly limited, and none works in quite the way that you envisage.
Basic Finder permissions settings depend on the user identity that you log on with, and cannot control individual folders in this way.
Extended permissions (ACLs) have much greater flexibility, but are still not intended to lock an isolated folder away behind a password.
There are plenty of utilities that allow you to turn a folder into something else, such as a disk image or an archive file, which can only be opened on entry of a password of your choosing. For instance you can create an encrypted disk image in Disk Utility: whenever you want to mount and access it, you will then have to enter the password. This forms the basis of Apple’s FileVault disk encryption, and is robust.
However that fundamentally changes the way in which you access the contents of your folder, as would the likes of encrypted Zip files using WinZip or another compression tool, or dedicated privacy systems such as PGP.
Another approach is that used by Hider 2, £14.99 from the Mac App Store, which can encrypt, hide, and password protect files and folders, but does so by locking them away in its vault.
Updated from the original, which was first published in MacUser volume 29 issue 12, 2013.