Nikon Dx 40 and I Mac?

Nikon Dx 40 and I Mac getting message can't import photo because a problem writing to the volume containing your I photo library
What does it mean

It means the hard drive on the iMac has an issue. In the Applications folder on the iMac, open the Utilities folder, then launch "Disk Utility" and "repair the drive". Do this with all the drives connected to the iMac.

The issue here is with iPhoto. Perhaps some settings have not yet been properly made. If you really must transfer the pictures now, all you need is a card reader to connect the camera's memory card directly to the computer's USB slot then use Finder to do the copying manually. This method solves all camera to computer connection problems. If you don't have a card reader, plugging the camera may still work with Finder.

Sounds like your IPhoto catalog is damaged.

1. Quit iPhoto if it is open.
2. Hold down the Command and Option keys on the keyboard.
3. Open iPhoto, but keep the keys held down until you are prompted to rebuild the library.
4. A dialog will appear with rebuild options. Select the options you want to use.
5. Click Rebuild to begin the rebuild process.

I have never heard of a Dx 40. Do you mean the D40x?

At any rate, your best solution would be to use a card reader to import the photos directly into the computer and put them in any folder you want to create for them.