This is a known bug in WHS Beta 2. It will continue to allow you to copy data there so long as your primary disk doesn't run out of space. There are many threads on this one though in both the forums and on Connect.
One example is:
https://forums.microsoft.com/WindowsHomeServer/ShowPost.aspx?PostID=1224586&SiteID=50
Go to that link, and do a find on the phrase:
'The primary reason has to do with a limitation in Drive Extender in beta 2.' -minus the quotes.
Hope that helps.