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How drives work

An illustrated primer for any drive: hard disks, SSDs, USB drives and memory cards; how partitions and file systems sit on them; what Windows does when you plug one in; and what can go wrong and be fixed.

Start here if your drive is failing

• Common situations: what to doDrive asks to be formatted, isn't detected, shows 0 bytes, clicks or is very slow: the most likely cause of each, and what to do first.
• Windows pop-ups on a damaged drive: what to clickWhat Windows' format, scan-and-fix and initialize prompts mean on a damaged drive, and which button keeps your files safe.
• When the file index is damagedWhy a drive turns RAW after a power cut or unplug, why your files are usually still there, and how they're recovered.
• Bad sectors, retries and hangsWhat a bad sector is, why one bad spot can freeze Windows for minutes, and how to read a failing drive without making it worse.
• The drive's health log (S.M.A.R.T.)What S.M.A.R.T. values like reallocated and pending sectors mean, why a failing drive can still say 'passed', and which numbers matter.
• How the safe copy worksWhy copying a failing drive once, easy parts first, protects your files, and how an image lets you recover from the copy instead.
• What can be fixed, and whenBoot sector, partition table, file index, bad sectors: what can be repaired, and why repairs should wait until your files are safe.
• Deleted, quick-formatted, fully formatted, wipedWhat deleting, quick formatting, full formatting and wiping change on a hard drive, SSD or memory card, and the chances for each.

How drives work

• The big picture: a drive is a stack of layersFrom numbered sectors to partitions, file systems and your files: the layers of any drive, and why 'format the disk' usually means one is damaged.
• Kinds of drivesWhat's inside each kind of drive, how each usually fails, whether deleted files can come back, and whether it keeps a health log.
• How a drive connects, and why it mattersWhy the same disk behaves differently in different USB enclosures, and what bridges and card readers mean for recovering data.
• Inside: platters vs flashHow hard disks and flash drives store data, why disk order matters on a hard drive, and why deleted data on an SSD is usually gone.
• Sectors, LBAs and clustersThe smallest units a drive and a file system work in: sectors, 512e and 4Kn, logical block addresses and clusters.
• Partition tables: MBR and GPTWhat MBR and GPT partition tables hold, which one has a backup copy, and why a USB stick with no partition table is normal.
• File systems comparedWhere each file system is used, how it finds files, what safety features it has, and how good the chances of recovery are.
• Inside NTFSHow NTFS finds your files through the Master File Table and folder listings, and why files can be recovered when parts of the MFT are lost.
• Inside FAT32 and exFATHow FAT32 and exFAT store folders and file chains, what deleting changes, and which deleted files come back whole.
• What Windows does when you plug in a driveDrivers, Disk Management, indexing and thumbnails: what Windows does to a drive on its own, and how to stop it straining a failing one.
• What has a backup copy, and what doesn'tWhich disk structures have backups (GPT, the NTFS boot sector, the MFT mirror) and which don't, so you know what can be rebuilt.
• EncryptionHow BitLocker and enclosure encryption affect recovery, where the keys are, and why some WD drives only read through their own USB bridge.

Reference

• Where DiskSalvor fitsHow DiskSalvor reads a drive as raw sectors, layer by layer, without writing to it, even when Windows calls the volume RAW.
• GlossaryPlain-English meanings of bridge, carving, CHKDSK, LBA, MFT, RAW, S.M.A.R.T., TRIM and other drive and recovery terms.