Limit the search to a subfolder or series of subfolders instead of searching all the disc volumes at once.
It’s recommended that you avoid searching in the System Folders, especially if you don’t have a good working knowledge of the Mac OS X operating system and its associated files.
If you use Apple’s Time Machine backup software, it’s recommended that you do not search a Time Machine backup volume, or delete files contained on a Time Machine volume. Deleting files from this location will impact Time Machine’s ability to restore your data.
While Tidy Up is capable of searching any mounted disk, keep in mind that doing so will be considerably slower than searching the computer’s internal hard drive.
To start with an optimized computer configuration, it’s always suggested rebooting the device. In addition, it is advisable (highly recommended if you have already experienced strange errors or it has been a long time since the last reparation) to repair the permissions and the disk using Disk Utility (/Applications/Utilities/Disk Utility) because they can prevent disk-reading and file-managing errors.