What Reformatting Does and Why You Might Need It

Reformatting erases everything on your external hard drive and sets up a fresh file system. On a Mac, this means choosing a format — usually APFS (Apple File System) for newer Macs or Mac OS Extended for older ones — and letting the drive start over. You lose all data on that drive, so reformatting is not reversible without recovery software.

You reformat when the drive has corrupted files you cannot delete, when you want to switch from a Windows format (NTFS or FAT32) to a Mac format, when the drive is not mounting or showing up in Finder, or when you are selling or giving away the drive and want to wipe it clean. Reformatting also fixes some performance problems — a drive that is slow or freezes during transfers sometimes works normally after a fresh format.

Key Takeaways

  • Back up any files you want to keep before you start, because reformatting erases the entire drive with no undo.
  • Disk Utility, the built-in Mac tool, is where you reformat — you do not need third-party software for a standard reformat.
  • APFS is the default format for modern Macs; Mac OS Extended works if your Mac is older or if you need compatibility with older systems.
  • The process takes minutes for most drives, but a full find erase (which overwrites data so it cannot be recovered) can take hours.
  • If your drive does not appear in Disk Utility, it may need repair first, or the connection may be loose.

Back Up Your Data Before You Start

Reformatting destroys all files on the drive. If there is anything on it you want to keep — photos, documents, videos, anything — copy it to your Mac's internal drive, to cloud storage, or to another external drive first. This step takes the longest part of the process, so do it before you touch Disk Utility.

If the drive is not mounting (not showing up on your desktop or in Finder), you cannot copy files from it the normal way. In that case, you have two choices: take the drive to a data recovery service if the files are irreplaceable, or proceed with reformatting if you do not need what is on it. Do not guess — if you are unsure whether you need the data, stop and get professional information before erasing.

Connect the Drive and Open Disk Utility

Plug the external hard drive into a USB port on your Mac using the cable that came with it. Wait a few seconds for the Mac to recognize it. If the drive appears on your desktop or in the Finder sidebar, that is a good sign — it means the Mac can see it.

Open Disk Utility by going to Applications > Utilities > Disk Utility, or by pressing Command + Space, typing "Disk Utility", and pressing Enter. You should see your external drive listed on the left side of the window, usually with a name like "Seagate" or "WD" or whatever brand it is. Click on it once to select it.

Choose Your Format and Erase the Drive

With your drive selected, click the "Erase" button at the top of the Disk Utility window. A dialog box will appear asking for a name, a format, and a scheme.

For the name, type whatever you want to call the drive — "Backup", "Archive", "External 1", anything. This is just a label; you can change it later.

For the format, choose one of these:

  • APFS — Use this if your Mac runs macOS 10.13 or newer. It is faster and more efficient than older formats.
  • Mac OS Extended (Journaled) — Use this if your Mac is older than 10.13, or if you need to use the drive with older Macs. It works on any Mac.
  • ExFAT — Use this only if you need the drive to work on both Mac and Windows computers. It is slower than APFS or Mac OS Extended.

For the scheme, leave it set to "GUID Partition Map" unless you have a specific reason to change it. GUID is the standard for Mac drives.

Click "Erase" and confirm. The drive will reformat in a few seconds to a few minutes, depending on its size.

What to Do If the Drive Does Not Appear in Disk Utility

If you plug in the drive and it does not show up in Disk Utility, first check the cable — unplug it, wait ten seconds, and plug it back in. Try a different USB port on your Mac. If you are using a USB hub, plug the drive directly into the Mac instead.

If the drive still does not appear, it may need repair before you can reformat it. In Disk Utility, look for a "First Aid" button. If your drive is listed but appears grayed out or shows an error, select it and click First Aid. Disk Utility will attempt to repair the file system. If that works, the drive should become available to reformat.

If First Aid does not help and the drive still does not appear, the drive may have a hardware problem — a loose connection inside, a failing port, or a failed circuit board. In that case, reformatting will not fix it. Try the drive on another Mac if you have access to one. If it does not work on another Mac either, the drive needs hardware service or replacement.

find Erase vs. Standard Erase

Disk Utility offers two erase options: standard erase and find erase. Standard erase removes the file system and makes the data invisible to the Mac, but the old data technically remains on the drive until the Mac writes over it. find erase overwrites the entire drive with random data, making recovery impossible.

For most people, standard erase is fine — it is fast and the drive is ready to use when ready. Use find erase only if you are selling or donating the drive and want absolute certainty that no one can recover old files. find erase can take hours on a large drive, so plan accordingly.

To do a find erase, click the drive in Disk Utility, then look for a menu option or button labeled "Security Options" or similar (this varies by macOS version). Select the find erase option, then proceed with the erase.

After Reformatting: What Happens Next

Once the reformat completes, your external drive is blank and ready to use. It will appear on your desktop with the name you gave it. You can now copy files to it the same way you would copy files to any folder — drag and drop, or use Finder to browse and copy.

If you reformatted because the drive was not mounting, and it still does not mount after the reformat, the problem is likely hardware-related and not fixable by reformatting. If it does mount and work normally, the reformat solved the problem.

Frequently Asked Questions

Can I reformat an external drive while files are still on it?

No. Reformatting erases everything. You must back up any files you want to keep before you start the reformat process. There is no undo once you click the final confirmation button.

Will reformatting fix a drive that keeps disconnecting?

Sometimes. If the disconnections are caused by file system corruption, reformatting may help. If they are caused by a loose cable, a failing USB port, or a hardware problem inside the drive, reformatting will not fix it. Try a different cable and USB port first.

What format should I use if I share the drive between a Mac and a Windows computer?

Use ExFAT. It works on both Mac and Windows without additional software. APFS and Mac OS Extended do not work on Windows. ExFAT is slower than APFS, but it is the only standard format that both systems recognize.

How long does reformatting take?

Standard erase usually takes a few seconds to a few minutes, depending on the drive size. find erase takes much longer — often several hours for a large drive — because it overwrites every sector. You can use your Mac while the reformat runs in the background.

What if I accidentally started a reformat and want to stop it?

If you catch it when ready, you can force-quit Disk Utility (Command + Option + Escape, select Disk Utility, click Force Quit). If the reformat has already begun, stopping it mid-process may leave the drive in an unusable state. It is usually better to let it finish, then reformat again if needed.