I shoot with a 4K camera and edit in Premiere. I recently purchased a WD MyBook hard drive for storing and working with video assets. I realized when it arrived that it is USB3 not Thunderbolt.
Can someone tell me if this drive is going to be significantly slower than a harddrive with Thunderbolt?
Actually, it won't be slower, because a Thunderbolt version of that drive wouldn't be faster. A 1 or 2 spindle solution (depending on which MyBook you purchased) is only going to hit 100MB/sec or 200MB/sec maximum regardless of the interface involved.
To really see a performance difference with Thunderbolt or SAS over USB-3, you either need a solution with 4 drives (spindles) or with SSD drives.