Did you go into disk management and initialize/format the drive? Once you do that it will show up in explorer.
Thank you! The D: was not on the RAID at all, but a "reserved" part of the un-initialized 1T drive (now drive F: ). I assumed that it must be a reserved section (which I can't say I saw on the previous machine, but have seen on laptops) on the main disc.
Have initialized, and do see! Double the capacity now, thanks again!
EDIT: Went in and removed the second "reserved" (there is one on the C: boot drive that I left) that was on the unallocated storage disc. Now it's the full amount available for storage and backup - thanks again!