You may have to update a disk driver in Sierra. Check the Seagate webpages with the exact type of your Seagate drive.
What Plug In are you referring to that allows Sierra to see the EHD?
Exactly how is that EHD formatted?
The plug in is called Paragon NTFS for Mac. I got it at the Seagate website.
Well if you updated/upgraded OS X recently it probably removed or disabled that piece of software. Mac OS X is famous for doing that. you might also need an updated version to work with the newest version of OS X.
But I suggest if you are going to stick with Apple Mac that you get another EHD and the first thing to do is format it in the Mac OS X format using Disk Utilities found in the Utility folder in the Applications folder. Then Copy all the data off the drive that is formatted in the Windows NTFS format over to that new Mac formatted drive. Then format the older EHD to the Mac format.
OS X can Read NTFS formatted drive but can't Write to it natively. That is why you needed that add on untility.