How to create a bootable drive on an external disk

Server.app is just a set of scripts and apps that run on top of the base OS X. So just follow the guides for 10.9 USB media.

So, I just carry a second USB drive with server.app and other downloads and use the first drive to install the OS, the second to outfit that image after it it running.

Assuming that you don't have an additional thumb drive (according to Apple with at least 12 GB - I got it to work with a 8 GB thumb drive) I recommend the following method:

• erase and repartition your WD easystore to two Mac OS Extended (Journaled) partitions - a bigger one first with the name "Sierra" and the size (total_size_of_external_drive - 12 GB) and a much smaller second one with the name "Sierra_Installer" and a size of ~12 GB. The partition scheme has to be GUID! If your MacBook's main partition already has the name "Sierra" choose another name for the first partition of the external drive. Below I assume the bigger volume on the external drive has the name "Sierra".

  Example: if your external drive has a size of 1 TB, then the first partition "Sierra" should have a size of 988 GB and the second "Sierra_Installer" partition one of ~12 GB.

• re-download the "macOS Sierra installer.app" on your MacBook
• open Terminal.app in /Applications/Utilities

• enter the command:

  sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Sierra_Installer --applicationpath /Applications/Install\ macOS\ Sierra.app
  

  hit the Enter key, enter your password (which won't be visible because it's masked), hit the Enter key again and let do createinstallmedia its job.

• Quit Terminal after the job is done and any other app.
• Reboot to "Sierra_Installer" (e.g. by choosing it in System Preferences > Startup Disk or by hitting the alt-key while booting and choosing Sierra_Installer)
• After a successful boot of the "Installer Boot Partition" install macOS Sierra to the volume "Sierra".
• After a successful installation configure Sierra.
• Finally remove the partition "Sierra_Installer" with Disk Utility and resize the volume "Sierra" to the full size of the disk.

The method proposed by user3439894 in a comment to your question is preferred though: create an Installer Thumb Drive, boot to it and install Sierra to a single-partitioned external drive.