Have a new MacPro, two Seagate thunderbolt sleds, a 240GB Seagate 600 SSD and a 1TB Crucial M550 SSD. The Seagate performs as expected. The Crucial is cripplingly slow (like, progress bar says an hour to copy 5GB.) Things I've tried: swapped sleds, drives, cables - no difference. plug the Crucial into an internal sata port on another machine - performs at full speed. connect the Crucial to the MacPro via a Seagate USB3 connector - performs at full USB3 speed. reformat the Crucial to exFAT - performs at full speed on Thunderbolt. So, specifically the only "slow" condition is the Crucial formatted with MacOS Extended on Thunderbolt. Does anyone have any idea what could be going on here? If I need to leave it formatted exFAT to get the proper speed, is there any downside to that?