How to have a non-obvious encrypted partition on an external hard-drive with MacOS Sierra?
How To Do It - Summary

What you need to do is prevent the file system from mounting that specific volume. This is typically handled by fstab, and configured via the /etc/fstab file. However, macOS (and OS X) specify a different command to correctly handle the editing of the /etc/fstab file - that is, vifs. Be warned that unless you have a default editor configured via the  EDITOR environment variable, the file editor which will be used is vi. There is additional information about vifs below, under the "Information About the vifs Command" heading, including how to use an editor other than vi.

Step by Step Instructions

The first thing we need to do is to get the UUID of the volume(s) we want to affect. To get the UUID, the drive must be connected to the computer, and volume must already be mounted. In this case, you would need to plug in the USB drive and enable access to the volume by entering the password.

Once this is done, which can be verified by seeing the volume in Finder, we need to get to the terminal. In the terminal window, type the command diskutil info /Volumes/&lt;Volume Name&gt; | grep 'Volume UUID'. In your case, you would replace "" with "Time\ Capsule". Note the backslash () is needed to escape special characters, such as space in this case. This should return a single line with a value similar to "Volume UUID:              AA2313FE-7C4A-340F-8293-D74EA0ED34DE". The UUID, which is "AA2313FE-7C4A-340F-8293-D74EA0ED34DE" here, is what we need. Select it, and copy it to the clipboard (&#8984;/Cmd+C).

We now need to edit the /etc/fstab file. By default, the 
For the scope of this answer, I will assume that you are comfortable with whichever editor you are using to perform the following steps.

Start the editor by typing sudo vifs in the terminal window. This will prompt for an administrator password to get administrative access to edit the file.

With the editor open, and the volume's UUID in the clipboard, we will append a line at the end of the file as follows: UUID=&lt;Volume UUID from clipboard&gt; /Volumes/Time\ Machine hfs rw,noauto. You can use the terminal command man fstab to get additional options or values to use in the entry, but here, the following options and values are specified as:  


/Volumes/&lt;Volume Name - from above&gt; Specify the mount point into which the volume is to be mounted.
hfs The file system type (Mac volumes are typically hfs - Hierarchical File System)  
rw,noauto  


rw Mount as read/write when mounting  
noauto Do NOT automatically mount the volume  



After you've added and updated the line(s) as necessary, save the file and exit the editor. To test that this has "taken", you can perform either a logout/login, reboot, or disconnect (eject) the disk and reconnect it. The volume should no longer be mounting or prompting for the password.

At a later time, when you need to mount the volume, you can return to the trusty terminal, and use the command mount /Volumes/&lt;Volume Name&gt;, or open Disk Utility, select the volume, and click the Mount button in the toolbar.

tl;dr


Get the volume's UUID, in the terminal, using the diskutil info /Volumes/&lt;Volume Name&gt; command
Use the command sudo vifs to properly (for macOS/OS X) lock and edit the /etc/fstab file
Add an entry to the /etc/fstab file for the volume, by its UUID, adding noauto to the mount options
To use/mount the Volume, use Disk Utility and the Mount options for the volume, or use mount /Volumes/&lt;Volume Name&gt;


Information About the vifs Command

Here is what you need to know about the what and the why of the vifs command. What this command does is simply lock the /etc/fstab file for editing to the calling user, and opens the file in vi (explaining the vi in vifs), or the editor (command) specified system's defined EDITOR environment variable. While we could directly edit the /etc/fstab file, using the vifs command is the only method recommended/approved by Apple.

References


Previous answer to a somewhat similar question  
Example on iDownloadBlog  
Apple's vifs documentation