Hi there, this trouble seems to happen due to the incompatibility of Toshiba Password Software with Apple OS X EI Capitan so you cannot access your Toshiba hard drive after the macOS update. Since the Toshiba team has released an updated version of the software, go to update the software on your device and then you can access the external hard drive for sure.
Go to the Toshiba website to download the newly coming software. Aside from that, you're allowed to access your external hard drive via a Windows PC for the files and documents stored on it as well.