What happens when you connect a new hard drive to your Mac

When you plug in a new external hard drive or install an internal one, your Mac will usually recognize it but won't let you save files to it yet. The drive needs to be formatted — which means writing a file system to it that macOS can read and write to. You'll also decide whether to use the whole drive as one storage space or split it into separate sections called partitions.

The most common choice for Mac users is APFS (Apple File System), which is the standard format for modern Macs running macOS 10.13 or later. If you're using an older Mac or need the drive to work with both Mac and Windows computers, you might use Mac OS Extended (also called HFS+) or exFAT instead. Formatting erases everything on the drive, so if there's anything you need to keep, back it up first.

Key Takeaways

  • New hard drives must be formatted before your Mac can save files to them, and formatting erases any existing data on the drive.
  • APFS is the default format for modern Macs, but Mac OS Extended and exFAT are options if you need compatibility with older systems or Windows computers.
  • Disk Utility is the built-in Mac tool for formatting and partitioning drives, found in Applications > Utilities.
  • You can create multiple partitions on one drive to organize files separately or run different operating systems, though most users keep a single partition.
  • After formatting, your drive will appear on the desktop and in Finder, ready to use for storing files or backups.

Opening Disk Utility and selecting your drive

The tool you need is called Disk Utility, and it comes built into every Mac. Open it by going to Applications > Utilities > Disk Utility, or search for "Disk Utility" using Spotlight (Command + Space).

When Disk Utility opens, you'll see a list of drives and partitions on the left side. Find your new hard drive in that list — it will usually say "External" or show a model name you recognize. Click on it once to select it. Do not click on any partition underneath it yet; select the drive itself at the top level. If you're not sure which one is your new drive, you can hover over each entry to see its size and connection type.

Choosing a format and partition scheme

Once your drive is selected, click the Erase button at the top of the Disk Utility window. A dialog box will appear asking you to name the drive and choose a format.

For the format, your options are:

  • APFS — Use this if your Mac runs macOS 10.13 or later and the drive will only be used with Macs. It's faster and more efficient than older formats.
  • Mac OS Extended (Journaled) — Use this if you have an older Mac or need to share the drive with someone using an older version of macOS.
  • exFAT — Use this if the drive needs to work with both Mac and Windows computers, or if you're sharing it across different operating systems.

For the scheme, choose GUID Partition Map if the drive will be used only with Macs. If you need Windows compatibility, choose Master Boot Record instead. Most users should stick with GUID Partition Map.

Naming your drive and erasing it

Type a name for your drive in the Name field. This is what will appear on your desktop and in Finder, so choose something you'll recognize — for example, "Backup Drive" or "External Storage". You can change the name later if you want, so don't worry too much about getting it perfect now.

Once you've chosen the format, scheme, and name, click Erase. Disk Utility will warn you that erasing will delete all data on the drive. If you're sure there's nothing on it you need, click Erase again to confirm. The process usually takes a few seconds to a few minutes, depending on the drive size.

Creating partitions if you need separate sections

Most people use a single partition for the entire drive, but you can split a drive into multiple partitions if you want to organize files separately or run different operating systems. To do this, select the drive in Disk Utility and click the Partition button.

Click the + button to add a new partition. You'll see a pie chart showing how the drive space is divided. Drag the dividing line to adjust how much space each partition gets, or type the size directly into the Size field for each partition. Give each partition a name and choose its format. When you're done, click explore. This will erase the drive and create the partitions you specified.

If you're not sure whether you need partitions, you probably don't. A single partition is simpler to manage and uses the full drive space without restrictions.

Verifying the drive is ready to use

After formatting completes, your drive should appear on the Mac desktop as an icon with the name you gave it. You should also see it in Finder under Locations on the left sidebar. Try dragging a test file onto the drive to make sure you can write to it. If the file copies successfully, the drive is formatted correctly and ready for regular use.

If the drive doesn't appear on the desktop or in Finder, or if you get an error when trying to copy files, the drive may not have formatted correctly. Go back to Disk Utility, select the drive again, and repeat the erase process. If the problem continues, the drive itself may be faulty.

Reformatting an existing drive to change its format

If you already have a formatted drive but need to change its format — for example, from Mac OS Extended to APFS, or to exFAT for Windows compatibility — the process is the same as formatting a new drive. Open Disk Utility, select the drive, click Erase, choose the new format, and confirm. This will erase all files on the drive, so back up anything you need first.

You can also use Disk Utility to repair a drive that's giving errors or behaving strangely. Select the drive and click First Aid instead of Erase. First Aid will check the drive for problems and fix minor issues without erasing your files. If First Aid can't fix the problem, you may need to erase and reformat the drive.

Frequently Asked Questions

What's the difference between APFS and Mac OS Extended?

APFS is newer and faster, designed for modern Macs and SSDs. Mac OS Extended is older but still works fine on any Mac. Use APFS unless you have an older Mac running macOS 10.12 or earlier, or you need to share the drive with someone using an older system.

Can I use the same drive with both Mac and Windows?

Yes, but you need to format it as exFAT. APFS and Mac OS Extended won't show up on Windows computers. exFAT works on both systems, though it's slower than APFS and has a 16 TB file size limit.

What if I accidentally selected the wrong drive in Disk Utility?

If you haven't clicked Erase yet, just click on the correct drive instead. If you've already erased the wrong drive, you may be able to recover the files using data recovery software, but there's no may provide. Always double-check the drive name and size before clicking Erase.

Do I need to partition my drive?

No. Most users keep a single partition for the entire drive. Partitions are useful only if you want to organize files into completely separate sections or run multiple operating systems on the same physical drive.

Why does my drive show as read-only after formatting?

This usually means the drive is write-protected, either by a physical switch on the drive itself or by file permissions. Check for a small lock switch on the drive and make sure it's in the unlocked position. If there's no physical switch, right-click the drive in Finder, select Get Info, and check whether "Locked" is checked under Sharing & Permissions.