HFS+

HFS+, sometimes called HFS Plus or simply Mac OS Extended, is basically Apple’s updated take on the old HFS file system. If HFS was the original foundation, HFS+ was the version that finally gave the Mac room to grow. It brought support for much bigger drives, longer filenames, and more modern ways of storing metadata, all of which the older system struggled with.

At its core, it still works in a similar way to HFS, but almost every limit that held the first version back was pushed out of the way. For years (from the late ’90s until APFS arrived) HFS+ was the file system you’d find on nearly every Mac.

HFS+ vs. HFS

We already went through the backstory of how HFS appeared and why Apple needed it (that’s all explained on the [HFS page]). If you’re curious about the historical side of things, it’s better to look there first. Here, we’ll focus on what actually separates the original HFS from HFS+ (Extended Hierarchical File System) in practical, technical terms.

• HFS had some pretty tight restrictions when it came to naming files. It stored names in the old MacRoman character set, and you couldn’t go beyond 31 characters, which felt cramped even back then. HFS+ switched to Unicode and allowed names up to 255 characters, so suddenly long descriptive filenames weren’t a problem anymore.
•  HFS had to use huge block sizes, sometimes 32 KB or even 64 KB, especially on larger disks. That meant a simple text file could end up wasting way more space than it used. When Apple moved to HFS+, the system started using much smaller blocks, often 4 KB.
• The catalog structure changed in a similar way. HFS relied on a fairly basic B-tree that worked fine for smaller disks, but it didn’t scale well. HFS+ reshaped the metadata layout so macOS could keep track of a much larger number of files without slowing to a crawl.
• Another big difference is how both systems handled disk size. HFS topped out at roughly 2 GB of practical volume support before fragmentation and block-size issues became unmanageable. HFS+ raised that into the terabyte range (theoretically up to 8 EB), which at the time made it effectively “future-proof.”
• HFS+ also added native support for things like file permissions, extended attributes, and more detailed timestamps, none of which were handled cleanly in the original HFS design.

HFS+ File Limits in Mac OS X

When Apple rolled out Mac OS X, the system was already built on top of the HFS+ file system, so whatever limits existed inside HFS+ ended up shaping the whole user experience. Since macOS depended on it for everything (starting the machine, storing apps, managing user files) any restrictions in how HFS+ Mac handled disk space or metadata became pretty hard to ignore.

• In terms of raw capacity, HFS+ can support volumes that reach multiple terabytes, so size isn’t the biggest issue. The trouble shows up elsewhere. Some parts of the format still rely on 32-bit values, which quietly puts ceilings on how far the file system can scale. You can work with very large files, but there’s a point where HFS+ starts feeling stretched.
• Another limit people bump into is the sheer number of files a single volume can hold. Because HFS+ organizes everything through B-trees, a heavily populated volume can slow down as those trees get dense.
• And then there’s the way HFS+ deals with allocation blocks and metadata. The design dates back to a time when drives were smaller and workloads were lighter, so once you start filling a disk with hundreds of thousands of items, the overhead becomes more noticeable.

None of these constraints were a big deal when Mac OS X first arrived, but as storage grew and apps became heavier, HFS+ slowly drifted out of its comfort zone. That mismatch between modern hardware and an older file-system design is one of the reasons Apple eventually moved on to APFS.

And if you’re dealing with an older Mac volume that already runs into directory corruption or missing files, which wasn’t uncommon on HFS and HFS+, you may want to check our guide on how to recover deleted files from HFS/HFS+ partitions, where we walk through recovery steps.

Structure of the HFS+ File System

The internal layout of HFS+ looks familiar if you’ve ever worked with classic HFS, but Apple expanded and rearranged several core components to make the system scale better on modern hardware.

• At the center of every HFS+ volume is the Volume Header, which acts like the file system’s table of contents. It stores pointers to all the major system files, the size and boundaries of the allocation blocks, and details about the volume’s state.
• Most of the real organization happens inside a handful of special files that Apple treated as part of the file system itself. The best-known of these is the Catalog File (a B-tree database) that keeps track of every file and folder. If the Finder can see it, the Catalog File knows about it. Its structure is similar to HFS, but HFS+ reshaped the record types and indexing so the system could handle a much larger number of items without slowing to a crawl.
• Whenever a file grows past what can be stored in a single record, HFS+ relies on the Extents Overflow File, another B-tree that keeps track of additional extents. Think of it as the place where HFS+ stores “extra pieces” of a file’s map, making sure the system knows where all the fragments ended up.
• HFS+ also maintains an Allocation File, a bitmap that records which allocation blocks are free or in use.
• And then there’s the Attributes File, which didn’t exist in the old HFS world. This is where HFS+ stores extended attributes, including Finder metadata, permissions-related information, and various OS-level tags that newer versions of macOS rely on. It allowed the system to store much richer metadata without breaking compatibility with classic tools.
• Finally, HFS+ volumes also contain a Startup File for older boot methods, though modern Macs rely on APFS and newer firmware paths instead.

Features of HFS+ (Mac OS Extended)

When people ask what is HFS+ or what is HFS+ format, they’re usually trying to understand why this file system stayed with the Mac for almost two decades. HFS+ may look like a modest update on top of classic HFS, but Apple packed in a number of under-the-hood improvements. And a lot of those improvements show up in the day-to-day features that shaped how macOS handled files.

• HFS+ switched from the old MacRoman encoding to full Unicode, allowing filenames up to 255 characters, which finally let macOS handle international characters, symbols, and long descriptive names without breaking compatibility.
• One of the biggest improvements was switching to 4 KB allocation blocks on most volumes. Classic HFS often needed 32 KB or even 64 KB blocks on large disks, wasting huge amounts of space on small files. HFS+ dramatically reduced this overhead, improving storage efficiency on every Mac.
• The move to 32-bit block addresses allowed HFS+ to scale into the multi-terabyte era. Although theoretical limits go even higher, in practice it provided all the space Mac OS X needed during its lifespan.
• HFS+ relies on several B-trees (Catalog File, Extents Overflow File, Attributes File) to track file system metadata. B-trees allow fast lookups even when a volume contains millions of files, something the older HFS catalog structure struggled with.
• A major stability feature. When enabled (and it was enabled by default in most versions of macOS), the Journal records metadata changes before they’re committed. This drastically reduces the risk of directory corruption after crashes, power failures, or forced shutdowns.
• HFS+ added proper support for both hard links and symlinks, which made it possible for Mac OS X to use complex directory structures and tools like Time Machine.
• HFS+ stores additional metadata (permissions, Finder tags, Spotlight data, etc.) in the Attributes File, replacing the old rigid resource-fork model. This allowed macOS to evolve into a modern Unix-based system while keeping its classic Mac features.
• HFS+ can store sparse files (files that logically take up a lot of space but physically occupy far less by not writing unused portions). This is useful for virtual machines, disk images, and developer tools.
• Even after transitioning away from classic Mac OS, HFS+ kept the familiar metadata structure: Finder icons, window positions, custom labels, type/creator tags, and Spotlight data all live comfortably inside the HFS+ framework.

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