I am not bitl but my guesses, in order of probability would be: * using filesystem that is not natively supported on one or both drives, like fuse ntfs on Linux

I am not bitl but my guesses, in order of probability would be: * using filesystem that is not natively supported on one or both drives, like fuse ntfs on Linux or ext3 on Mac. * copying over slower network like wifi * lots and lots of small files especially in a single directory * usb 2.0