[dnanian]

Understood. The Console messages don't really help, actually -- we're aware of the problem.

So, to work around the issue, create the sparse image, of the appropriate size, using Disk Utility. You'll only have to do this once.

1. Open Disk Utility (found in /Applications/Utilities).
2. Without selecting any volume/drive on the left side, click the New Image toolbar button.
3. Give the image an appropriate name (like Backup), and store it on a drive other than the one you're trying to back up.
4. In the Size pop-up, choose Custom.
5. In the sheet that appears, enter the size of the drive you're trying to back up. Note that the pop-up does not have a selection for GB: you need to enter "40000" MB for 40GB. Click OK when you've got the right size entered.
6. Decide whether or not you want the image encrypted, and choose the proper value in the Encryption pop-up.
7. In the Format pop-up, choose sparse disk image.
8. Click Create.

Once it's created, go back to SuperDuper! and choose the "Disk Image..." selection in the destination pop-up. Navigate to the location where you created the image and click it (it's grey, but that's OK -- click it anyway), then click "Save".