Whenever the Macintosh mounts a disk after a crash or an improper shutdown, the operating system (ROM) performs a limited disk check to determine if there is damage. A bug exists in this disk check routine that keeps the disk from mounting. If the disk is the boot disk, the Macintosh will display a blinking question mark, otherwise, the Finder will display a dialog indicating that the disk is not a Macintosh disk, and let you eject or initialize the disk..
Specifically, if the Extents Tree (a file on disk that keeps track of file fragmentation) is equal to or larger than 786,432 bytes and close to a multiple of 65,536 bytes, and there are a certain number of fragmented files on the disk, the disk check routine will report that the disk is damaged the next time the Macintosh mounts a disk after an improper shutdown.
The problem can also occur if the Catalog Tree (a file on disk that keeps track of the locations of files) is greater than 4,587,520 bytes and close to a multiple of 65,536 bytes. This would most likely only be possible on a large file server volume with more than 25,000 - 30,000 files and folders.
It is important to note that the disk is actually okay. The disk check routine just thinks it is not a Macintosh disk.
What to do if you encounter the Disk Check bug:
The only way to get the disk to work again is to use a disk recovery program to properly mount the disk, such as CP DiskFix or Safe & Sound.
How to prevent the Disk Check bug:
If the bug occurs due to the size of the Extents Tree, the easiest way to avoid the Disk Check bug is to regularly defragment the files on the problematic disk with an optimizing program like CP Optimizer.
We have not found an easy way to avoid the bug if the Catalog Tree is the problem other than to reduce the number of files or folder on the volume.
A more difficult solution is to backup and re-format the disk with a slightly different partition size (as a rule of thumb, +/- 100K bytes of the current size). This will change the size of the Extents Tree and Catalog Tree and make the bug less likely to occur. Is it imperative to backup the disk before re-formatting because re-formatting erases all data. After re-formatting the disk, use Disk Bug Checker to find out how susceptible the new partition is to the Disk Check bug. Unfortunately, there is no way to find out what exact partition size to use because most disk formatting programs work differently.
About the Central Point Disk Bug Checker:
Disk Bug Checker is designed to tell you how susceptible your hard drives are to the bug. Simply run the program, click your drive in the main window, and click Check. Disk Bug Checker displays a dialog to tell you how likely the bug is to occur.
The top part of the dialog tells you how many extent records you can have before the bug occurs. The bottom part of the dialog tells you how many catalog records you can have before the bug occurs.
The number of records in the catalog at any given point in time is determined by the following: 1 record for each file on the disk, 2 records for each folder on the disk, and some ‘file thread’ records that are sometimes used by aliases.
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