Firstly, please note that due to Apple's sandboxing rules, Disk Doctor is not allowed to scan anything other than your main drive (i.e. the one that has your OS X installation on it). This is a rule set by Apple and until they change their policy, we won't be allowed to support external drive scanning.
If by mistake you managed to select the wrong User folder to scan, please follow the instructions below:
1. Quit the Disk Doctor app
2. Open Terminal
3. Paste in the following command: defaults delete com.fiplab.diskdoctor
4. Press the Enter key to submit the command
5. Reopen the Disk Doctor app