I duplicated my WD 500GB HDD to @TB Seagate SSHD using Vantec NexStar HDD Duplicator.

The problem originates in your choice of cloning system.

Most stand-alone duplicators can do cloning, but have no real smarts or options you can customize. The result is that the clone copy you make will be exactly the same as the original in all aspects, including the sizes of all the original Partitions. I can see that is not what you expected.

I suspect what you want is that the Partitions on the original 500 GB unit should be reproduced in some cases, and upgraded for others. More specifically, any "hidden" system Partitions (things that contain backups, a recovery Partition, etc. that will never be changed) should be the same on the resulting larger HDD. However, the C: drive Partition should be expanded to take up all of the available space on the new 2 TB unit. Right?

To do this you need to run cloning software on your machine and not depend on the limited abilities of a duplicator. PLUS, you need to read the documentation that comes with it.

Since your new HDD is from Seagate, get their Disk Wizard software utility package (free from their website). It appears to be a customized version of Acronis True Image and will make a clone only TO a Seagate HDD. It does many things more than that, though. Read the manual document that comes with it, especially the chapter on cloning, because you will need to use the menus to make changes.

I presume that your original 500 GB HDD still has all your stuff on it, and you just want to get the new SSHD re-done as you expected. I suggest you re-install your old 500 GB HDD and connect it to the same SATA port it was on before. That way when you boot up it will boot from that HDD. Make sure the Disk Wizard software is installed on that old HDD's C: drive. Now install the new SSHD (probably already in your machine) and connect it to a different SATA port. Run the Disk Wizard under Windows and make SURE you set the NEW SSHD as your Destination device - all stuff there will be destroyed, and you do NOT want to do that to any other HDD! First, wipe out all the Partitions already on that SSHD so it is all Unallocated Space. Now start the cloning operation. BUT do NOT merely approve what it says! You will be presented with a plan of how it will do the job, including particularly the sizes and specs for the several Partitions it plans to make. The sizes will be in proportion to the sizes of your old ones on the smaller drive, and you do not want that. Use the menu systems to change the plan so the the backup partitions etc. are the SAME size as before, or marginally larger. Then make the new C: drive Partition use all of the rest of the space. Most of the other options you could customize will be set correctly (C: Partition bootable with the NTFS File System, probably MBR-style Partitioning, use Full Format option). When you have that set up exactly as you want, let it proceed and do something else - it will take hours and you need patience. When it is done, back out of the Disk Wizard and shut down.

I recommend now that you open the case and swap some SATA data cables so that the NEW SSHD is now plugged into the SATA port that used to be for the old smaller HDD. That way when you turn the machine on it will find the (new) bootable drive where it always was and boot just fine. For the old 500 GB unit, I suggest you disconnect it completely. Leave it that way as an untouched perfect backup copy of your old drive, until you are SURE that the new SSHD really did get everything. THEN you can decide what to do with the 500 GB unit and re-install it, clean it off, and use it.