External hard drives and SSDs on a Mac: a guide

External drive problems at a glance

How fast is my external drive?

For a rough idea of a drive’s write speed, use Terminal’s dd command: it writes a large test file and prints how fast it went.

1. Open Terminal. It’s in Applications > Utilities, or search for it with Spotlight.
2. Type this command and press Return. It writes a 4 GiB file to your home folder:

dd if=/dev/zero of=~/speedtest.tmp bs=1m count=4096

3. Read the last line: the speed in bytes per second. Divide by 1,000,000 for MB/s.
4. Delete the file: rm ~/speedtest.tmp

studio@MacBook-Pro ~ % dd if=/dev/zero of=~/speedtest.tmp bs=1m count=40964096+0 records in4096+0 records out4294967296 bytes transferred in 0.731498 secs (5871455718 bytes/sec)studio@MacBook-Pro ~ % rm ~/speedtest.tmpstudio@MacBook-Pro ~ % 

To test an external drive, put the file on it: of="/Volumes/Studio Archive/speedtest.tmp", using your drive’s name. The cable matters as much as the drive. Apple’s USB-C Charge Cable carries data at up to 480 Mbps (USB 2.0), at most 60 MB/s, while its Thunderbolt 3 (USB-C) Cable supports up to 40 Gbps. How to test SSD speed on a Mac explains why a dd result is only a rough number.

Why isn’t my external drive showing up?

Most missing drives are a connection or a setting. Work through these in order:

1. Plug the drive straight into your Mac, without a hub or extension cable. Try another port, then another cable. If it has a power adapter, plug it in.
2. On a Mac laptop with Apple silicon, allow the accessory when your Mac asks. If you click Don’t Allow, your Mac won’t recognize it, and asks again the next time you connect it.
3. In the Finder, choose Finder > Settings, click Sidebar, then select External disks.
4. Open Disk Utility and choose View > Show All Devices. If the drive is there but grayed out, select it and click Mount.

If it won’t mount, run First Aid on it in Disk Utility, then try Mount again. External drive not showing up on a Mac? Fix it also covers drives your Mac doesn’t see at all.

What does “Disk Not Ejected Properly” mean?

“Disk Not Ejected Properly” means the drive left your Mac before it was ejected: it was unplugged, its cable came loose or it lost power. To disconnect a drive safely, eject it first, in any of these ways:

• Click the Eject button next to the drive’s name in the Finder sidebar.
• Select the drive, then choose File > Eject, or press Command-E.
• On the desktop, drag the drive’s icon to the Trash.

If it won’t eject, an app may still be using a file on it: quit the app or close the document, then try again. For a Time Machine backup disk, Apple says to eject it or shut down your Mac before you disconnect it. If the alert appears when you didn’t unplug anything, connect the drive directly, try another cable and port, and give it power. “Disk Not Ejected Properly” on a Mac: why it happens covers drives that keep dropping out.

Which format should an external drive use?

A format, or file system, is how a drive organizes its files. Apple describes these:

• APFS: the default for Macs with macOS 10.13 or later. It’s optimized for SSDs and can be used with hard drives too. Choose it for a drive that stays with Macs.
• ExFAT: compatible with Windows, for drives over 32 GB. MS-DOS (FAT) is for 32 GB or less.
• Mac OS Extended (Journaled): the file system of macOS 10.12 and earlier. Apple’s Disk Utility guide for macOS Tahoe no longer lists it, but Time Machine still supports backup disks that use it.

To see a drive’s format, open Disk Utility, choose View > Show All Devices, select the drive’s volume, then click Info.

Time Machine can’t back up to a disk formatted for Windows. Changing a format erases the drive, so copy your files off first. APFS, exFAT or Mac OS Extended: which format to use walks through the choice and the Erase steps.

Can I move files to an external drive to free up space?

Yes. Apple suggests it when your Mac runs short of space: media such as a Photos library can use a lot of storage, and it can live on another drive.

1. Connect the drive, then open a Finder window.
2. Select the files you want to move and drag them to the drive, listed below Locations in the sidebar.
3. Open one or two on the drive to check them. Then delete the originals from your Mac and empty the Trash: their space isn’t free until you do.

Keep a second copy of anything you can’t replace, such as in a Time Machine backup. How to free up space on a Mac: every way, ranked shows which files are worth moving first.

How do I check an external drive’s health?

Many disks include Self-Monitoring, Analysis and Reporting Technology (S.M.A.R.T.) hardware, which lets a disk test itself for potential problems. Disk Utility can show its status: the disk’s general working condition.

1. Open Disk Utility and choose View > Show All Devices.
2. Select the drive itself, at the top of its list in the sidebar, then click Info.
3. If you see a message that the disk has a fatal hardware error, back up as much as you can and replace the drive: Disk Utility can’t repair that.

If there’s no hardware error but the drive still acts up, run First Aid on each volume, then each container, then the drive itself. If Disk Utility says the disk is about to fail, back up your data and replace it. Apple notes that Disk Utility can’t detect or repair every problem a disk may have.

Questions people also ask

Can a Mac use a drive formatted for Windows?

Yes. Disk Utility supports ExFAT and MS-DOS (FAT), the file systems compatible with Windows, so a Mac can use a drive formatted either way.

Can I use an external SSD for Time Machine?

Yes. Time Machine works with a USB or Thunderbolt drive connected directly to your Mac. APFS or APFS Encrypted is the preferred format, and a disk formatted for Windows can’t be used until it’s erased in a Mac format.

Why does my Mac ask me to allow a drive?

Mac laptops with Apple silicon ask before a new USB or Thunderbolt accessory can connect. To change when it asks, choose Apple menu > System Settings > Privacy & Security, click Accessories, then use the menu next to Allow accessories to connect.