Disk First Aid: "HFS Wrapper Partition Damaged" Error

With Mac OS 8.1 and later, Mac OS can support both Mac OS Standard and Mac OS Extended formatted volumes. Mac OS Standard formatted volumes are sometimes called HFS volumes. Mac OS Extended formatted volumes are sometimes called HFS Plus volumes. Users can tell how a volume is formatted by selecting the volume in the Finder, and choosing Get Info from the File menu. The type of volume appears in the Format field.

Most HFS Plus volumes, including those created by Drive Setup and the Finder's "Erase Disk..." menu item, are really a type of hybrid. They are a HFS volume with a "pure" HFS Plus volume occupying most of the space on the HFS volume. The HFS volume is "wrapped around" the real (embedded) HFS Plus volume. That's why it is called a HFS wrapper. The HFS wrapper is not a separate partition, and both the HFS wrapper and the "real" or "pure" HFS Plus volume inside it are in the same partition, assuming the disk actually uses partitions.

The HFS wrapper functions

• It allows Mac OS 8.1 and later to start up from a HFS Plus formatted volume. The ROM of every computer shipped since the Mac Plus could recognize and mount HFS volumes, but not HFS Plus volumes. This means the ROM sees the HFS wrapper as an ordinary HFS volume, mounts it, and tries to start up from it. To make that happen, there is a tiny System file in the wrapper that contains just enough code to find the real System file in the embedded HFS Plus volume, and continue starting up from there.
• There is a Read Me file in the wrapper. The name is normally "Where_have_all_my_files_gone?". If a user tries to mount the volume on a system without HFS Plus support, anything before Mac OS 8.1, this is the only file visible. It is a SimpleText file that explains that this is a HFS Plus volume, and that they need Mac OS 8.1 or later to use the volume. This was designed this way to prevent the "This is not a Macintosh Disk. Do you want to erase it?" alert users would otherwise see.

In some cases, Disk First Aid 8.2 and later may report the following error message:

"The HFS wrapper partition is damaged. You should make backup copies of your files, erase the volume, and restore."

Disk First Aid can and does fix some kinds of damage to the HFS wrapper. Users should only see the above message if Disk First Aid could not fix the damage. If the error message appears everytime Disk First Aid is run on the volume, back up any important files, erase the volume and restore the software.

Although this type of damage is rare, the HFS wrapper can be damaged by having power removed from the computer when it is in use or after a crash when using incompatible software.