macOS · Storage & Backup
Manage External Drives On Your Mac
External Storage covers how macOS handles USB, Thunderbolt, and network-attached drives connected to your Mac — formatting them, choosing a file system, ejecting safely, and deciding which ones are eligible for Time Machine backups. Disk Utility is the main tool for managing these drives.
How to open External Storage
- Connect the external drive via USB, Thunderbolt, or your network.
- Open Disk Utility (Applications → Utilities → Disk Utility) to view, format, or repair the drive.
- Select the drive and choose Erase to format it, picking a file system that matches your needs.
- Eject the drive from Finder's sidebar before physically disconnecting it.
What you should know
- Format external drives as APFS, Mac OS Extended, or exFAT depending on how you'll use them.
- Erase, partition, or repair a drive's directory structure using Disk Utility's First Aid.
- Choose whether a connected external drive can be selected as a Time Machine backup disk.
- Safely eject drives from Finder or the menu bar before physically disconnecting them.
Always eject an external drive properly before unplugging it — disconnecting without ejecting can corrupt files or the entire disk's directory structure.
Why it matters
External drives are where most people keep backups, large media libraries, or overflow storage, so understanding formatting and safe ejection prevents avoidable data loss and corrupted disks. Choosing the wrong file system can also make a drive unreadable on other computers you need to use it with.
Best practices
- Format drives as APFS for Mac-only backup or storage use, or exFAT if you need cross-platform compatibility.
- Run Disk Utility's First Aid periodically on drives you use heavily to catch directory errors early.
- Keep at least one drive dedicated solely to backups rather than mixing backups with everyday file storage.
Common problems and fixes
- An external drive doesn't appear in Finder or Disk Utility.
- Try a different cable or port, check Finder → Settings → Sidebar to confirm external disks are set to show, and restart the Mac if it still isn't detected.
- macOS says the drive isn't readable and needs to be formatted.
- The drive's directory structure may be corrupted or it's formatted for a different OS; try Disk Utility's First Aid first, and only reformat if repair fails and you don't need the existing data.
Tips and tricks
- Hold the mouse button down at startup, or use the Startup Disk pane, to boot from an external drive if it contains a bootable macOS installer.
Frequently asked questions
Can I use the same external drive on both a Mac and a Windows PC?
Yes, if you format it as exFAT, which both operating systems can read and write to; APFS and Mac OS Extended are not natively writable on Windows.
Why does macOS warn me before ejecting a drive that's 'in use'?
It means a file on that drive is still open in an app; close the file or app first, then eject to avoid losing unsaved changes or corrupting the disk.
Related settings
- Storage ManagementStorage settings show a visual breakdown of exactly what's using space on your Mac's disk, plus built-in recommendations — like storing files in iCloud or automatically emptying the Trash — to free up space without manually hunting for large files yourself.
- Time Machine BackupTime Machine automatically backs up your entire Mac — apps, accounts, preferences, and files — to an external or network drive, so you can restore individual files or your whole system if something goes wrong.
- Startup DiskStartup Disk lets you select which internal or external bootable volume your Mac uses when it powers on, useful if you have multiple macOS installations, a bootable backup, or a Windows partition via Boot Camp on an Intel Mac. Apple silicon Macs manage most of this through Startup Security Utility instead.
More Storage & Backup guides for macOS
- Optimize StorageOptimize Storage is a set of Apple-recommended actions — like storing files in iCloud, auto-emptying the Trash, and removing watched TV shows and movies — that free up local disk space with minimal effort. It's found inside About This Mac's Storage settings.
- iCloud DriveiCloud Drive keeps your files in sync across every device signed into the same Apple ID, and can automatically include your Desktop and Documents folders so they're backed up and accessible from any Mac, iPhone, or iPad. Files sync in the background and are accessible from the Finder sidebar or iCloud.com.
- Documents & Desktop iCloud SyncAutomatically upload the contents of your Desktop and Documents folders to iCloud Drive so they stay backed up and accessible from other signed-in devices.
- Migration AssistantMove apps, files, user accounts, and settings from another Mac, a Time Machine backup, or a Windows PC onto your current Mac using the built-in Migration Assistant utility.
- Disk Utility: Partition & Format DrivesUse Disk Utility to create or resize partitions, format internal or external drives to a specific file system, and erase disks before reuse.
- iCloud+ FeaturesEnable and configure premium iCloud+ features such as iCloud Private Relay, Hide My Email, and HomeKit Secure Video storage tied to your paid iCloud storage plan.