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It is more likely an HFS/HFS+ file system bug
Authored by: jollyroger on Apr 04, '03 08:10:09AM

If your hard drive ever reached the point of being over 85%-90% full, then it is likely that the HFS file system caused the extents tree to be overwritten (overlapped) because the B-Tree portion of the directory itself was out of space!

Take a look at the comments in this MacFixit forum:

<http://www.macfixitforums.com/php/showflat.php?Cat=&Board=Forum8&Number=435249&Forum=All_Forums&Words=&Match=Entire%20Phrase&Searchpage=0&Limit=25&Old=1day&Main=435138&Search=true>

Pay attention to MacWizard's comments (he is/was on the TechTool team). Some quotes:

" HFS+ does a much better job of keeping track of files that have many pieces (extents). If an HFS disk is heavily fragmented and more than 70% full, irreparable damage can take place to the Extents B-Tree portion of the disk directory if any more files are added. The corresponding limit for HFS+ disks is 85%."

and

"If the disk is more than 85% full, and is sufficiently fragmented (something I cannot precisely define), the Extents B-Tree is in danger of irreparable damage. When new entries need to be written in the Extents B-Tree, older ones are simply overwritten. The developer of TechTool Pro has reproduced this problem in a lab setting.

The Extents B-Tree keeps track of file extents (pieces) beyond the first three extents of each file (those are recorded in the Catalog B-Tree.) Within Apple, the Extents B-Tree is called the Extents Overflow file. The name alone suggests a potential problem. I suggest you archive some of the data as soon as possible.

I have screenshots of disks that have zero free bytes and still work, but they were not heavily fragmented. "

I'm not sure I can remember where or when, but at some point, years ago, I learned that you should never fill a Mac HD more than 90% or you will have problems. Since then I have only let my hard drives get 90% full on occasion. And I do seem to have much fewer directory-related hard drive problems than other Mac users.



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