This affects sharing TO W10 from other machines -- NOT FROM W10 TO other machines which seems to work just fine.
Ms has changed some security aspects of file sharing over networks in W10. The changes only affect W7 and earlier Windows OS'es and most Linux distros using SAMBA.
You need to BLOCK INBOUND port 139 on your WINDOWS 10 machine if you use something like NetBIOS to access your shared machines on a network. If you use an LMHOSTS file (I don't as I don't want fixed IP addresses on a network) or actual IP addresses to access your shares then you don't need to do anything.
(The two ports in question here are 139 and 445 - only block TCP port 139).
Note - you need to block the INCOMING port on the WINDOWS 10 Machine. If you don't have the firewall enabled then you will still get problems sharing via hostname - so enable the firewall (Windows).
What apparently is happening is that Windows 10 is only listening for PORT 445. SAMBA / W7 machines check Port 139 first -- wait for an acknowledgement (or other response) - and then if negative send out port 445. On W10 port 139 doesn't return any acknowledgment so on Linux SAMBA times our while Windows issues the message Cannot access .... etc.
This fix might not work in the final W10 release but it's still working even in build 9879.
Remember the problem is sharing TO Windows 10 -- NOT FROM WINDOWS 10 and only from Linux (samba) or Windows releases 7 and earlier.
(For those using Storage spaces - you have to share at FOLDER / DIRECTORY level as there's no "Physical Volumes of course).
If you are using CENTOS 7 as a Host you may still find that you can SEE the shares FROM Windows but can't access them -- for some reason you need to set wins = yes in the samba conf file if you are using a typical home LAN (not domains etc).