External Disk Reformatting

I have an external disk formatted as APFS. I would like to erase and reformat it as MacOS Extended. Disk Utility does not give me that option. Is there some other way I can accomplish that?

What does “Scheme” mean? My choices are GUID Partition Map, Master Boot Record, and Apple Partition Map. I believe I normally choose GUID Partition Map, but what are the advantages and disadvantages of each? Thanks.

Storage devices do not directly contain volumes. They contain partitions. Most disk formats (HFS+, FAT, ext, etc.) consume the entire space of one partition.

APFS is different in that its container fills one partition. You can then create multiple APFS volumes within a single container. (And if there are two or more volumes in a container, they will share the same pool of free space.)

This is for most storage devices you are likely to use today (e.g. hard drives and SSDs). Small devices like floppies and thumb-drives may be formatted without partitioning, in which case one volume fills the device. I know this can be done for FAT volumes and in the past, HFS (before Apple dropped that format). I don’t know if HFS+ can be used without partitioning the media. I would be very surprised if APFS can.

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For an external drive that will only be connected to a Mac, GUID Partition Map is best. Master Boot Record is the old Microsoft DOS/Windows partition scheme for boot drives in particular.

See Partition schemes available in Disk Utility on Mac - Apple Support


As I wrote previously, most non-trivial storage devices (especially hard drives and SSDs) must be partitioned before you can create volumes on them. This requires creation of a “partition table” somewhere on the device in order to define where each partition starts and ends. These “schemes” are different kinds of partition tables.

  • Apple Partition Map (APM) is the one Apple used on Macs starting from their first hard drive (the old HD20) and is required for any volume bootable on a PowerPC or 68K-based Mac. I think it is also used by hard drives on Apple II systems running ProDOS. It uses 32-bit numbers to refer to the disk blocks where partitions begin/end. This means a theoretical maximum of about 4 billion blocks. Since most storage media uses 512 byte blocks, this imposes a limit of 2 TiB per storage device. Devices with larger block sizes can be used for larger capacity, but Mac OS may not be able to support such devices without special system extensions.

  • Master Boot Record (MBR) was invented by Microsoft/IBM for hard drive support on MS-DOS, starting in version 2.0. There have been a few variations of MBR over the years, but like APM, it uses 32-bit block identifiers and so also can’t handle normal storage devices larger than 2 TiB. Storage devices with larger block sizes can allow larger capacities, but operating systems typically require special device drivers to support such devices.

  • The GUID Partition Table (GPT) is a part of the Universal Extended Firmware Interface (UEFI) and (among other things) avoids the capacity limits of APM and MBR partitioning schemes. It uses 64-bit block addresses for a maximum capacity (with 512 byte blocks) of 8 ZiB (that is, about 8 billion TiB), which is expected to be large enough for all storage devices in the forseeable future. [citation needed]

In general, if your storage device is larger than 2 TiB, you must partition it using the GPT scheme, because the other two schemes can’t handle devices that large.

Additionally, if you need to boot an Apple Silicon Mac from the device, then it must be GPT, regardless of capacity. If you need to boot an Intel Mac from the device, then it also should be partitioned using GPT. (I’m not certain, but I think it might be possible to boot some older versions of macOS on Intel Macs using APM-partitioned media, but it is not a recommended configuration.)

And, FWIW, if you want to boot a PC with a 64-bit version of Windows, the boot volume must also be partitioned with GPT (in addition to having a motherboard that supports UEFI).

If, however, you are making a boot device for PowerPC or 68K Macs, then it must be partitioned with APM.

If the device you’re formatting is not going to be bootable (that is, only holding data volumes), then you can use any of the three partition schemes. Just keep in mind that any device larger than 2 TiB must be partitioned with GPT or you will be limited to only accessing the first 2 TiB of its capacity. If you think you might need to use it with older computers whose system software doesn’t recognize GPT, then you might want to use APM (if it will only be used with Apple computers) or MBR (which should be supported by any PC and most versions of Mac OS).


I ran into this confusing problem myself not long ago.

  1. In Disk Utility, go to the “View” menu, and make sure that “Show All Devices” is selected instead of “Show Only Volumes”.
  2. Once you have “Show All Devices” selected, you should be able to select the USB drive or its top-level APFS container and see all the formatting options.

Here is a short video that runs through the steps:

By the way, I’ve seen Gary Rosenzweig’s MacMost website mentioned on TidBITS before, but I don’t know if I’ve actually visited his site before today. He has an impressive collection of videos on his YouTube channel, ranging from the basics to expert-level explanations. Recommended!


Thank you, for the answer and the reference.

Thank you for the detail. Today, the only reason I would have for not using GPT is that I have a PowerBook G4 that I last used a few years ago, and it is remotely possible that I would want to move files to or from it.

Thank you, josehill! This is exactly what I was looking for!

If you want to boot that G4 from the drive, then it would have for be partitioned with APM. But if you just want to read files, GPT might be OK, depending on what version of macOS you’re running.

Intel processor support was introduced in Mac OS X 10.4.4 (and some earlier “Developer Transition Kit” builds). I believe GPT support was introduced there. At least it was for Intel Macs - I’m not sure if PPC Macs running 10.4 could access GPT-partitioned devices.

When Mac OS X 10.5 shipped, I believe all installations could access GPT-partitioned devices.

So if your PowerBook G4 is recent enough to run Mac OS X 10.4.4 or later, you might not need APT partitioning for a device to be readable.

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Mac OS 10.4.11 and 10.5.7 on internal drive (APM) can access an external GUID drive as I have an old Mac Mini for music listening and the music files are on an external Toshiba drive using GUID.

If the user still needs Classic OS9 access in 10.4, then the drive needs to be bootable as it was originally using APM.

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You got me curious, so I fired up the PowerBook G4. It’s running 10.4.11 and could mount an APFS volume on an external SSD with GPT. Thanks.

APFS??? Are you sure? APFS support was introduced (in its earliest and not final form) in macOS 10.12.4 (Sierra). 10.4.11 shouldn’t have a clue what to do with it.

But if that GPT-partitioned SSD has HFS+ or FAT volumes, then that makes sense, and I’m glad to hear it works.

Like Shamino said, that’s not possible. If it’s an APFS volume, it’s going to say it’s not readable and ask if you want to format, eject or ignore.

My external GUID volumes formatted as HFS+ are recognized by 10.4.11 as I mentioned earlier.

You’re right, of course. I assumed that the bottom entry in Disk Utility’s window was for the volume of interest, since it I had just connected it.

The SSD that the G4 recognized has a GUID partition map with a “USB External Physical Volume • Mac OS Extended (Journaled)” volume, which I assume is HFS+. (And I wonder why I did that, since G4 compatibility was certainly not on my mind.)


Glad to hear that it’s recognized. It’s great that we can still use some of these older machines with newer external drives. I still have my PowerBook G4 and a few Mac Mini G4’s which are used daily for music listening and other light tasks and still work fine after all these years.

Yes. Apple has renamed their file systems over the years, but “Mac OS Extended” is what Apple calls HFS+ these days.

The most interesting (at least to me) Apple file systems, supported by various Macs over the years are:

  • Macintosh File System (MFS). The first file system Apple invented for the Mac. Used primarily on 400K floppies although can, in theory, work on hard drives up to 20MB. Key features are that it’s a flat file system - all files reside in the root directory, with folders being an artifact of the GUI, not related to the way the files are stored on the media. This makes performance very bad on large media, making it inappropriate for anything larger than a floppy disk. Apple made MFS support read-only in System 7.6 and dropped it in Mac OS 8.

  • Hierarchical File System (HFS), also known as “Mac OS Standard” or “HFS Standard”, was introduced in System 2.1 as a part of Apple adding hard drive support, HFS has actual subdirectories (hence the “hierarchical” in the name). Apple dropped support in Mac OS X 10.6 (Snow Leopard), but keeping it as read-only until macOS 10.15 (Catalina), 35 years after its introduction.

    Although HFS is not very useful today, Apple dropping read-only support has been problematic, because quite a lot of classic Mac CD-ROMs are formatted with HFS, and are no longer readable without third-party software. Older Mac floppies (especially 1.44M disks, which can be accessed via USB drives) are typically formatted with HFS, because HFS+ has too much overhead to be practical on such small storage devices.

  • HFS+, also known as “Mac OS Extended” or “HFS Extended”, was introduced in Mac OS 8.1 and continues to be supported today.

    Apple designed HFS+ with future expansion in mind, so they did not need to replace it with a new file system in order to support various Mac OS X features, including:

    • Unix file system features (e.g. permissions, hard links and symbolic links)
    • Journaling
    • Case-sensitive filenames
    • Unicode 3.2 decomposition (replacing the 2.1 algorithm previously used)
    • Access control lists (via previously-reserved “inline attribute data records”)
    • Directory hard-linking (to support Time Machine)
    • Compressed files
    • Volume encryption (FileVault 2)
  • Apple File System (APFS) was introduced in macOS 10.12.4 (Sierra) and is Apple’s preferred file system today. It is probably the best to use on SSD media, but has significant performance problems on mechanically-accessed media (e.g. hard drives).

  • ProDOS. This file system was created for the Apple III (and it’s SOS operating system), but was also used for ProDOS on the Apple II series.

    Mac systems 4 though 7.1 included the Apple File Exchange utility, which could copy files to/from ProDOS- and PC DOS/FAT-formatted floppies. Systems 7.1.2 through 9 included PC Exchange (under various names) capable of actually mounting ProDOS and FAT floppies.

    PC Exchange was dropped in Mac OS X, where support for FAT-formatted media became a natively-supported file system, but support for ProDOS media was dropped.


10 posts were merged into an existing topic: HFS access on modern Macs