Problems getting macOS Sierra on a bootable hard drive I'm trying to get a bootable hard drive with macOS Sierra going so that I can use Octane Render. My Macbook is running macOS High Sierra, which is apparently buggy with Octane at the moment. 

I purchsed a WD easystore, partitioned it to be Mac OS Extended (Journaled), and put the macOS Sierra installer on it (I downloaded this from the App Store). First, I was starting up with that disk and continuously getting the following error message: 
"This copy of the Install macOS Sierra.app application is damaged, and can't be used to install macOS."

NOW I'm not even getting to that stage, the drive isn't even showing up as a bootable disk! When I go to System Preferences > Startup Disk, it's not even there as an option.

Please help me - getting verrrry frustrated as I have no idea what I'm doing. Thanks in advance!
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.