So with the upgrade to Android 7.0, you may notice in the release notes (to Shield 5.0.2) this little tidbit in the release notes :

Added ability to write to attached USB or SD card storage.

They also have a forum post where they go in-detail into the process for setting up external drives or SD cards as internal media.

This post, however, is about how to write files to External drives connected to the ShieldTV over the network, specifically where you have read/write access and where apps have read/write access.

As of Shield 3.2, you could read both the internal drives and the external drives by accessing the "Access SHIELD folders on PC" -- however, you could only write files to a specific location:

\Android\data\com.plexapp.mediaserver.smb

So, you could put all your media files there and Plex would have read/write access while other apps (SPMC, e.g.) would have read-only access. So if you realize you have a duplicate media file on your USB, you could either delete it from your PC over the network-share or try to delete it from within Plex (not from within SPMC even if that's your preferred MC app). You could not write to:

\Android\data\com.semperpax.spmc16\files

over the network fileshare in order to have your media in a place SPMC would have both read/write access (and you still can't)

Now, though, you can do something even better. After your upgrade, you will see on your externally-connected HDs a new folder:

\NVIDIA_SHIELD

It appears that you can (over the network fileshare) read/write to this folder (like you could the \Android\data\com.plexapp.mediaserver.smb folder as of version 3.2). The NVIDIA_SHIELD folder, however, also appears to be writable to SPMC. So now you can rename / delete files from within the SPMC app if you move you store your media in the NVIDIA_SHIELD folder of your HD.

So (finally) after having the box for two years, we can add/modify media from a PC to the Shield and modify/delete that media from SPMC or Plex or whatever program you want to use.