What is the Macintosh Extended Format, and how does it compare to the Macintosh Standard Format?

The Mac OS Extended Format (HFS+) is a volume format compatible with Mac OS 8.1 and higher, including Mac OS X. In terms of volume, folder, and file size limits, it is similar to the Macintosh Standard Format (HFS). However, because it uses a 32-bit address to store data, HFS+ can assign more than four billion allocation blocks to a volume, vastly more than the HFS limit of 65,356. As a consequence, the minimum file size for even very large volumes remains small, usually around 4KB. With some HFS+ formatting utilities, the minimum file size is as little as 512 bytes (i.e., a single logical block). If you are still using HFS on your system and would like to switch to HFS+, consider the following issues:

• Because you do a low-level format when you reinitialize a volume into HFS+, you must back up your data. Some utilities can turn an HFS volume into HFS+ without erasing files (e.g., Alsoft's PlusMaker). However, if your computer crashes during the initialization process, you will lose data.
  
  
• You cannot use HFS+ to initialize a volume that's smaller than 32MB (e.g., a standard floppy disk). In addition, volumes that are smaller than one gigabyte do not greatly benefit from it.
  
  
• Versions of Mac OS prior to 8.1 cannot access HFS+ volumes unless they are mounted over the network. Also, 68k Macintoshes cannot use HFS+ volumes as their startup disks.
  
  
• Unless a disk utility program (e.g., Norton Utilities, TechTool Pro) says it supports HFS+ or the Mac OS Extended Format, do not try to use it on your HFS+ volume. Otherwise, you risk losing data or even causing volume damage.
  
  
• Compared to HFS, which causes greatly degraded performance when folders containing more than a few hundred files are opened, HFS+ manages large folders very well. However, because there are many more allocation blocks on an HFS+ volume, I/O is sometimes a little slower. The result is a speed trade-off between the two formats.

HFS+ supports Unicode and filenames of up to 255 characters, but you will need to use Mac OS X to take advantage of this capability. Additionally, Mac OS X 10.3 and higher supports journaling for HFS+, which provides protection against power outages and hardware component failures, reducing the need for repairs.

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