Do you currently have read/write access to the folder in question? I am also using Outllook 2007 with Windows 7 64bit and have two Outlook data files, one on my Win 7 partition and one on my old XP partition. I don't have any problems with accessing and saving to both.
The XP data file was a pre-existing Outlook file, but I needed to give my username read/write access to that drive before things worked properly, so you may want to check that.