The internal drives on the Windows machine still test at their full speed. I cannot for the life of me fathom what has changed, everything is still enabled on the windows side, and crucially this is only for externally mounted drives. So, if you wish to eschew the cloud and share Windows 10 Tech Preview files with your Mac OS X Yosemite box on a home or local network, follow these steps. However, I have connected again a couple of weeks later and the network speeds have dropped back down to 5-6MB/s. I ran across this page and when I enabled NetBIOS over TCP/IP this seemed to fix the issue - external drives mounted on the Win side would test at their capacity of 320MB/s when testing from the MBP. However, external drives did not work - I was getting write speeds of around 5MB/s They connected up nice and easily, and when running a test of disk speed from the MBP (with Blackmagic disk speed test) the internal drives on the Win machine tested at their full speed for around 500MB/s.

I was trying out a direct connection between my MBP and consumer Win10 machine connected direct via thunderbolt.

Share windows directory for mac