Also what is the formate of the USB stick? have you tried a different formate? like journal, i asume you are using fat 32, but if you are using NTSF and a program to write to it on mac that program might be the problem. MAC's are capable of reading these but may not be as good their more designed for PC. Whan you conect the drive does it read a USB 2.0 conection or a USB 3.0?
-go to the apple icon in the top left, than about this mac, than more information, than system report, than scrol down and click USB if the device reads conected as 5Gbs (which is 500mbs the capital letter means the actual speed is one decimal place over) than it is conected as USB 3.0. Also i would try to conect a differnt USB 3.0 device like a WD mybook which gets around 135-165mbs transfer speeds (mine is a PC version reformated for journal, i can't see spending the extra 20-80$ to have a drive pre-formated, which only takes a few secounds using disk utility. Just check out the conection status, and the formating type for now, i doubt they put a usb 2.0 chip in a new mac pro.my mac pro from 2011 has a SATA 600mbs controler, they are usually known for being smater like alothough the computer itself was capable of SATA 600mbs the drive was rotational and was only sata 300mbs which mis still more than enough for any rotatioanl drive on the market. They only put the SATA 600mbs so we the consumers could put an after market SSD in the computer later.