How to connect an extra monitor and a thunderbolt (LaCie) harddrive on a MacBook Pro (13" mid 2012)?
If either your LaCie Thunderbolt Drive offers more than one Thunderbolt port ([https://www.lacie.com/se/products/product.htm?id=10621](like this one)), or you have a native Thunderbolt display, you can "daisy-chain" them:


connect one device to your macbook (the one with two Thunderbolt ports, e.g. the drive)
connect the other device (e.g. your display) to the second Thunderbolt port of the first device (e.g. your drive)


If none of your Thunderbolt devices support daisy-chaining (i.e. don't offer a second TB port), then you would need a Thunderbolt Hub.

Another possibility would be to connect the hard drive via USB, if it can, [https://www.lacie.com/se/products/product.htm?id=10599](like this one).

One caveat though: this should work for any thunderbolt display. If you want to use the DisplayPort adapter, you should check whether it works when it is not connected directly to your Mac.