How do I format an external drive?

Formatting is straightforward and irreversible, and the two decisions that matter are made before you start: what will the data be, and what will need to read it.

Safety

Step by step

1. Copy everything off first and check it.Not just copy — open several files from the copy. Formatting removes the index immediately, and recovering afterwards is expensive and uncertain.
2. Decide what needs to read the drive.This is the whole decision. Windows only, Mac only, both, or a television, camera or car stereo as well. The last case usually forces exFAT or FAT32 regardless of anything else.
3. Choose exFAT for cross-platform use.Reads and writes on both Windows and macOS, and handles large files. The right default for a drive that moves between machines. Slightly less robust to sudden disconnection, so eject it properly.
4. Choose NTFS for Windows only.Windows-native, journalled and reliable. macOS can read it but not write to it without extra software, so it is the wrong choice for a shared drive.
5. Choose APFS for Mac only.The right choice for a Time Machine drive or Mac-only storage. Windows cannot read it at all without third-party software, and Windows will offer to format it — which is where people lose data.
6. Avoid FAT32 unless something requires it.It cannot hold a file larger than 4GB, which rules out most video. Some older devices and car stereos still need it, and that is the only reason to choose it.
7. Format on Windows through File Explorer.Right-click the drive, choose Format, pick the file system and a volume label. For anything more — repartitioning, a drive with no letter — use Disk Management.
8. Format on macOS through Disk Utility.Select the physical drive rather than the volume beneath it, choose Erase, then pick the format and the scheme. GUID Partition Map is the right scheme in almost every case.
9. Understand quick versus full format.A quick format rewrites the index in seconds and leaves the data physically present. A full format checks every sector and takes hours. Use full if the drive has been misbehaving, quick otherwise.
10. Know that neither securely erases.A quick format leaves everything recoverable with ordinary software. If the drive is being sold or given away, use a secure erase or full-disk encryption before formatting.
11. Give it a meaningful label.Not the default. It appears on every machine it is plugged into, and three drives called 'Untitled' is a problem you create once and live with.
12. Eject properly from then on.Writes are cached, so pulling the cable mid-write corrupts the file system rather than one file. This matters more with exFAT than with the journalled formats.
13. Do not format a drive that is failing.If it is clicking, disappearing or throwing errors, formatting makes recovery harder. Get the data off first, then decide whether the drive is worth keeping at all.

Tips

• Windows offers to format any drive it cannot read, including perfectly healthy Mac-formatted ones. That offer is the most common cause of accidental data loss with external drives.
• For a Time Machine backup, let macOS format the drive during setup rather than doing it yourself — it picks the right scheme.
• Label the drive physically as well as digitally. A marker pen on the case saves plugging in three drives to find one.

Common mistakes

• Formatting a drive Windows says is unreadable — Windows says that about any format it does not understand, including healthy Mac drives. Read it on the system it was formatted for instead.
• Assuming a quick format erases the data — It removes the index and leaves the data physically present, recoverable with free software. Selling a drive after a quick format hands over its contents.

If it doesn't work

Questions people ask