File system
A file system is the method a computer uses to organise files and folders on a volume, and to record where each file's data is stored.
The storage device holds a long run of sectors. The file system is the set of records written on top of those sectors: names, folders, and the addresses that say which sectors belong to each file. NTFS, APFS, ext4, HFS+, FAT32, and exFAT are file systems. The disk itself is not a file system.
How does a file system work?
When a program saves a file, the operating system asks the file system for free space, writes the bytes into sectors, and updates the index. When a program opens a file, the file system looks the name up and turns it back into sector addresses. When a file is deleted, the file system typically marks its index entry as deleted rather than erasing the data, so the bytes stay on the device until the space is used for something else. On a solid-state drive that uses TRIM, the drive can erase the freed blocks instead, and trimmed blocks may read back as zeros.
Why can file system damage hide files that are still there?
The sectors may still hold the file contents when the index is corrupt, formatted, or replaced: the computer just can no longer look the names up. That is a logical failure of the file system. It is not, by itself, proof that the device has stopped working.
What should you do if the computer wants to format the disk?
Do not format it. Formatting writes a new, empty file system over the volume. Decline the prompt when a volume you were using suddenly appears as raw, or when the computer says the file system is corrupt. A disk check that offers to repair the volume also writes to it, so a sector copy comes before that repair if the files matter.
What might you notice?
- The computer asks you to format a disk that was in use
- A volume that used to have a name now shows as raw
- Folders are listed, but the files inside will not open
- The disk was formatted, or a partition was deleted