I have had the same problem running on Vista Home Premium. Once I save an NEF the first time, if I try do change any of the edits or add an edit and re-save I get that error message. I wrote to Nikon about it in Ocober and they put me through some exercises which didn't solve the problem. Then I found this thread on dpreview http://forums.dpreview.com/forums/read.asp?forum=1021&message=28577634.
So it's a common problem and probably not related to Win7 64. I forwarded the link to Nikon and they had never heard of it(!). They should check some of the more active forums to see what's going on.
Try right clicking on the drive (or folder) and then choose security. If you don't already have full permissions set, choose edit and change the option to allow full control. In my experience, more often than not, that is the source of this problem after a Windows 7 upgrade.
Tried that and it wasn't Read Only. I had been using NX2 and had not had this problem, then it arose and I haven't been able to get rid of it. I uninstalled and reinstalled NX2 and I went back to V2.2.0. Nothing helps.
Since it appears to be a common problem it looks like a program glitch. I don't know what triggers it but it seems to be persistent once it appears.
In anticipation of receiving a replacement laptop from Dell, I called Nikon tech support to find out if NX2 is compatible with Windows 7 64 bit. The tech said Nikon doesn't expect it to have it compatible until the end of December. I called three days ago.
My gut feeling says that it relates to the ownership of the files - not to NX2. If on that 1 Tb drive you also have other files, like a Word file, you could check this by trying to open/edit/save it. If you have the same issue in Word then it is defintely an ownership issue you can find here (http://support.microsoft.com/kb/308421/en-us ) at the bottom how you can make yourself the owner of the folders. This describes it for XP, but in W7 it shouldn't be much different - at least you get a hint. I hope this helps.
I went to the root directory and changed file permissions and this solved the issue. Using file manager I chose properties, then the security tab, and then I gave myself all access rights. The confusing thing -- and I've yet to have time to read up on win7 users -- is the large number of default users. I just assumed that as the sole user I would have all rights.
I am happy it worked. I am planning migrating to Win7 as well - with a very similar set-up (keeping the disk with all files in tact), and I anticipated already that I would get a similar issue like you. I just wait with migrating until NX2 is 64bit and Win7 proof - things still work great on XP so I wait patiently (and lazily to be honest). But enjoy Win7!
When I installed Windows7 onto my now dual boot computer, I ran into this problem frequently as most of the folders, files etc. contained on the XP64 system would kick out this error. After a little research I found this little fix for the problem. http://www.sevenforums.com/tutorials/1911-take-ownership-shortcut.html Just download and install this little registry fix and every time you run into the problem, just right click and it will fix it right up for you