i just bought one usb3 sandisk extreme 64GB, at first the read & write speed were like 30 ~ 40 MB/s almost the same on both usb2 and usb3 ports, i thought it was strange i formatted the disk to NTFS quick format, but the speed didnt change,
i solved this problem by updating my motherboard drivers like intel chepset, intel rapid storage, bios others ,
after installing the drivers i restarted my pc, then the speed jumped to read speed around 150 ~ 250 MB/s
write speeds around ~ 150 MB/s
i was like whaaaat speed.
i think the problem most of the owners of this sandisk extreme usb3.0 is with drivers, formatting does virtually nothing.
i hope this helps.
