iMac 27" Thunderbolt

I have iMac 27" mid 2011 OSX LION 10.7.5 I have 2 Lacie 6tb thunderbolt 2 hard drive iMac not connection by thunderbolt to the hard drives but will connect by usb3 cable. Drives work when connect to Mac Bookpro by thunderbolt I have checked on system report it said Port no devices connected Link status 7 and firmware 2.1.1. Need to buy more Hard drives for a large project and need more speed

Thanks