Floppy Disk Compatibility and Incompatibility in the Mac World

Earlier today in the Apple Macintosh Enthusiasts Facebook group, Charles Lott asked if an OS X Mac with a USB floppy drive could write disks that a Mac running System 7 could use. The short answer is, it depends.

400K Disks

3.5" floppy diskettesThe original Mac floppy disk format was MFS, for Macintosh File System, and it is only used for 400K single-sided floppies – the only kind of floppy drive supported by the original Macintosh, the Mac 512K Fat Mac, and the Lisa 2 (a.k.a. Macintosh XL). These drives had a variable speed motor that allowed the Mac to pack 400K into a disk that would only hold 360 KB on a fixed-speed drive. (This is also the reason non-Mac computers can’t mount 400K and 800K Mac floppy disks.)

Macs with 800K double-sided drives and Mac-compatible 1.4MB High Density (HD) drives can read and write the MFS format used by 400K disks as long as they are running System 7.5.5 or earlier. Under Mac OS 7.6 and 7.6.1, they can read 400K floppies but not write to them. There is no support at all for 400K floppy disks in Mac OS 8 and beyond.

There is also no support for floppy disks with greater than 400K capacity in those original three models. Disk access is controlled by code in the system ROMs, and that can’t be updated to support double-sided drives. (There is also a 20 MB maximum volume size for MFS hard drives.)

All Mac OS versions up to System 7.1 support formatting 400K floppy disks.

800K Disks

When Apple introduced the Mac Plus in January 1986, it adopted double-sided floppy drives with 800K capacity – and a newer disk format known as HFS, for Hierarchical File System. (HFS had been introduced in 1985 to support Apple’s first Mac hard drive, the 20 MB Hard Disk 20, which connected via the slow floppy disk port. Those with pre-1986 Macs had to boot from a floppy drive that would install the HFS drivers that allowed them to boot from the HFS formatted hard drive.) MFS disks use a flat file system. Although the Mac makes it appear that it has true folders, this is an illusion.

With HFS, the Mac gained a multi-level hierarchy of folders and the ability to access hard drive volumes at huge as 2 TB. Keep in mind, this was the age of 20-40 MB hard drives, 1 GB drives were a long ways off, and its only in recent years the 1 TB drives have become commonplace.

Except for the pre-1986 Macs that have built-in 400K floppy drives, all Macs running System 3 through Mac OS 9.2.2 and using an Apple or specifically Mac-compatible floppy drive can read 800K floppy disks. (Mac OS X does not support internal floppy drives.)

Macs running System 3 through Mac OS 9.2.2 support formatting 800K floppy disks.

1.4 MB Disks

Mac II with color displayApple introduced high density (HD) floppy disks to the Mac with the Mac IIx in September 1988. Going forward, all new Macs with floppy drives would have what Apple sometimes called FDHD (floppy disk, high density) or SuperDrive – not to be confused with the DVD-burning optical drive of the same name.

Not only that, but the Mac SE was updated with the HD drive, and Apple offered upgrade kits for both the Mac II and pre-FHDH Mac SE. Because their system ROMs did not support these drives, the 1986 Mac Plus and 512Ke do not work with Apple’s HD floppy drives.

Apple SuperDrive floppy drives use a variable speed motor, making them compatible with 400K and 800K floppy disks – as long as the operating system also supports them.

Any Mac with a built-in floppy drive introduced since September 1988 has an FDHD, and the March 1987 Mac SE and Mac II may have been updated for FDHD as well. Every version of the Classic Mac OS from System 3 through 9.2.2 can read, write, and format 800K and 1.4 MB floppy disks as long as the mechanism is an Apple FDHD or a third-party Mac-compatible drive with a variable speed motor.

Macs with high-density Apple floppy drives and System 3 through Mac OS 9.2.2 support formatting 1.4 MB floppy disks.

Mac OS 8.1: HFS+ Makes More Efficient Use of Disk Space

As hard drives grew in capacity, some limitations of the HFS format became apparent. In the era of small hard drives, it didn’t matter, because whether you were using a floppy disk, a hard drive, or a removable media (SyQuest, Zip, etc.) drive, they all used the same size block of data – 512 Bytes (0.5 KB).

Or did they? Another limitation of HFS is that it cannot work with more than 65,535 files or blocks of data. That meant the operating system would have to cluster more than one 512 byte chunk of data into an allocation block. For instance, on a 1 GB partition, space was allocated in blocks of 16 KB, using 32 of those 512 byte data blocks.

Apple addressed this by introducing the HFS+ file system with Mac OS 8.1 in January 1998. HFS+ supports over 4 billion allocation blocks. That means that a 500 MB drive or partition will still use 512 Byte allocation blocks, a 1 GB drive will double that to 1024 Bytes (2 x 512), and so on.

Under HFS, that began to happen once hard drives passed the 30 MB mark, so HFS+ made for much more efficient use of data space. Below 32 MB of so, HFS and HFS+ both use 512 byte allocation blocks.

Alsoft created PlusOptimizer to convert HFS hard drives to HFS+ format.

And what does this have to do with floppy disks? Keep reading, because HFS+ floppies became a possibility in the era of USB floppy drives!

Macs Without Floppy Drives: The iMac Generation

1.4 MB Only (Sort of)

Bondi iMacOne of the chief faults of the iMac when Steve Jobs unveiled it in May 1998, according to most critics, was its lack of a built-in floppy drive. This was especially true for people who had an iMac at school or work but an older Mac or PC at home. You had to buy a USB floppy drive to read the disk from your other computer.

Problem is, none of these USB floppy drives have the variable speed motor necessary to read the Mac’s 800K disks, so you had to be sure to use 1.4 MB HD floppies (or 720K floppies in the case of PCs with 3.5″ floppy drives). Fortunately all but the oldest Macs support HD floppies, but the expense of an external USB floppy drive was discouraging to many longtime Mac users – and potential iMac adopters.

Another option was to use Apple’s software to format a double-sided, double-density floppy to 720K instead of 800K, in which case most USB floppy drives can read and write to it. On the other hand, it’s a pain to use with Macs, which don’t expect that format.

Mac OS X Changed Everything

When Apple introduced Mac OS X, one thing it didn’t do is provide drivers for the Mac’s internal floppy drives. It does support USB floppy drives, and it would normally format them as HFS disks – although there were some changes over time.

Siber-Sonic, who used to work for Apple, did extensive research of this subject, answering the question, “If I format a HD floppy disk in a USB floppy drive, what format with OS X use? Also, do all versions of OS X support HFS floppies?”

HFS Floppies: Fully Readable, No Writes with OS X 10.6 or Later

The good news is that all tested versions of Mac OS X are able to read 1.4 MB floppy disks in a USB floppy drive. The bad news – well, at least less good – is that starting with Mac OS X 10.6 Snow Leopard, you can’t write to an HFS-formatted floppy disk. To do that, you need to use OS X 10.5 Leopard or earlier.

HFS+ Floppies: Full Compatible with All Versions of Mac OS X

Under OS X 10.6 or earlier, the default format when formatting a floppy disk is HFS, but earlier versions (at least back to 10.4 Tiger) let you choose HFS+ as your disk format.

1.4 MB floppy formatted as HFS

1.4 MB floppy disk formatted as HFS has 1.4 MB available space.

1.4 MB floppy disk formatted as HFS Plus

1.4 MB floppy disk formatted as HFS+ has 1.3 MB available space.

Would you have guessed that an HFS+ floppy would have less available storage space than an HFS disk? Roughly 123 KB of additional space is used by the file system. Regardless, at least you have a floppy disk you can use in current Macs as well as vintage ones running Mac OS 8.1, the first with HFS+, and later.

What About More Modern Macs and Mac OS Versions?

Siber-Sonic found that under OS X 10.8 Mountain Lion, although the operating system can read and write HFS+ floppies, the only options for formatting are the FAT and ex-FAT formats used in the Windows world. That said, they can still read those HFS floppy disks from 1988!


In short, 400K MFS floppy disks can only be used in Apple branded and Mac-compatible 400K, 800K, and 1.4 MB drives. You can write to them through System 7.5.5 and read them through Mac OS 7.6.1. You cannot access them at all in Mac OS 8.0 or newer

800K floppy disks also require an Apple branded or Mac-compatible floppy drive with a variable speed motor. These can be used in any Mac with an 800K of FDHD floppy drive with System 3 through Mac OS 9.2.2. They are not compatible with any version of Mac OS X.

1.4 MB HFS floppy disks require an Apple FDHD, Mac-compatible HD floppy drive, or USB floppy drive. They are supported in all versions of the Classic Mac OS from 3.0 through 9.2.2 and can be read in any Mac. You can write to them using System 3.0 through Mac OS X 10.5 Leopard. Later versions of OS X can read but not write to them.

Finally, 1.4 MB HFS+ floppy disks can be fully accessed from Mac OS 8.1 forward, although OS X 10.8 Mountain Lion and later will not let you format an HFS+ floppy. OS X 10.6 and 10.7 will let you erase an HFS floppy, but they will only format floppy disks as HFS+.

Further Reading

Keywords: #macfloppy #hfs #hfsplus

Short link: http://goo.gl/plGnYa