Disk Copy is a useful utility which copies an image of either your Internal disk, or an SSD, into a file on either an attached PC, or your Internal disk, or an SSD.
Why? Well when you delete a file, the contents of the file remain on your disk. There is a special part of the disk called a File Allocation Table which stores information on all the files on your disk. When a file is deleted, this FAT is changed, but the data remains on your disk. You can then use this program to copy the entire contents of your disk into a file, and then copy the data you accidently deleted.
It's much easier to restore text files this way, because you can open the image file in a text editor, and cut and paste anything you recognise.
This program is the beginning of an Undelete program which will automatically undelete files, both plain text and binary. Undelete will be able to use files produced by DiskCopy to restore files, so if you have lost a binary file, you should copy the disk, and save it for when Undelete is finished.