Need help how to connect 2 external monitors to iMac (27- inch late 2013)

I have a iMac 27" Late 2013 and am wanting to upgrade to connect (2) external 27" monitors. I should point out this is the model before the 5K came out and has the 1st generation thunderbolt connection, 2 connections total.

Here is what I am trying to accomplish:

I would like to connect (2) external 27" monitors to where each monitor will be independent. Being iMac is the main screen then external monitor one is screen two and external monitor two is screen three. The external monitors will most likely be 4K 27" monitor that way I am set for future upgrades. I know I will not be able to have them at 4K resolution currently which is fine. The problem is this, I have 2 thunderbolt connections on the iMac, one being used by a MyBook Hard Drive. The only way I can find where I can setup both monitors to run independent is to connect each monitor to each thunderbolt connection using a mini display to thunderbolt adaptor. But I then need to figure out how to connect the MyBook Hard Drive up with both Thunderbolt connection used. Can I connect one of the monitors to the thunderbolt connection of the Drive and achieve a video connection? Is there a thunderbolt splitter for my generation of iMac that would treat each monitor independent and not a mirrored setting?

Looking for suggestions