External Hard Disk Drive Not Showing Under 'Devices'
The 'Sharing and Permissions' option has been changed to 'Read-Only' for no apparent reason. This has happened to me previously, but it still showed up under 'Devices' in the Finder window.

I believe in the macOS Sierra update, this does not show.

Fix for issue:
Download the Paragon Driver for Mac OS to gain access for Seagate external drives without having to reformat from the following link:

http://www.seagate.com/gb/en/support/downloads/item/ntfs-driver-for-mac-os-master-dl/

Install the Paragon Driver and follow the steps. You will be asked to restart your Mac. Once your Mac has been restarted, the external HDD will display once again under 'Devices'.

Note: This resolution allows you to have access to your external HDD to modify it as you wish. It does not reformat the HDD - so your data is safe.