Before you can use a hard drive the first time, it will need to be formatted. Most hard drives come pre-formatted from the factory, so you may never need to format a drive manually. Sometimes when a hard drive fails, the computer will mistakenly ask you to format the drive, thinking it is a new drive. This is almost always a mistake.
Stop!
There is no benefit to formatting a hard drive that has failed. In almost all cases it will make the recovery more difficult, or less successful. In some cases it can make the data impossible to recover. Tip: Never format a drive you want to get data from.
What is a hard drive format?
It’s easiest to think of a hard drive format as the file system it uses. Mac and PC drives use different types of format to store the data. Windows uses the NTFS filesystem while macOS / OS X use HFS+ and recently APFS. Formatting a drive is a bit like a factory reset and makes the drive appear blank again.
If you format a hard drive containing data, the index gets removed and replaced with an empty one. The computer now thinks the disk is empty and will allow you to write data back to all areas of the drive (like a new drive). If you don’t write anything back to the drive then most of the old data should still be recoverable. It may not be possible to get the data back with the original file and folder names, but the files should be OK. If you do write new data to the formatted drive, it will overwrite some or all of the old data. (As a basic example – If you had 100 pictures and wrote back 5 new ones after formatting, we should be able to recover the remaining 95.)
Exceptions.
There are some exceptions to this rule. Crucially, any files that reference other files by name, or that pull in data from multiple sources may not work without the original structure. An example of this is an Adobe InDesign file, which usually contains links to external images & graphics. The external images are not copied into the InDesign file, but referenced by filename as required. The file may request something like “Work/Project/logo.jpg,” however after a format recovery, all jpg files are named like 00001.jpg, 00002.jpg, 00003.jpg and so on. Unless you manually search through all the files and rename the correct one to logo.jpg, the InDesign file is pretty useless.
Other similar examples are Garageband files, Logic Pro and FinalCut Projects. Although Garageband files do contain all the media files, they are actually stored in a bundle, which is just a special type of folder. These package files are also broken when a drive gets formatted. Example from inside a Garageband file below.
If you have formatted a hard drive, or been prompted to format by the computer, there is a good chance that most of the data should be recoverable. Most of the damage to formatted drive comes after this point. It is really important to stop using the drive, and don’t write anything more to the drive.