Path: utzoo!mnetor!uunet!husc6!think!ephraim
From: ephraim@think.COM (ephraim vishniac)
Newsgroups: comp.sys.mac
Subject: Re: HFS Flames
Message-ID: <13951@think.UUCP>
Date: 18 Dec 87 18:00:10 GMT
References: <3580@husc6.harvard.edu> <3600@husc6.harvard.edu>
Sender: usenet@think.UUCP
Reply-To: ephraim@godot.think.com.UUCP (ephraim vishniac)
Organization: Thinking Machines Corporation, Cambridge, MA
Lines: 21

In article <3600@husc6.harvard.edu> olson@endor.UUCP (Eric K. Olson) writes:
> [discussion about file tags, their merits and costs]
>
>Floppies (MFS for sure and maybe HFS) when they're formatted have two
>directories, but I believe the file system only updates one of them for
>speed.

When a volume is initialized, a copy of the volume header (sector 2) is
placed just short of the end of the volume, in the next-to-last sector.
Even this is some use for scavenging, as it retains basic info about the
original state of the disk: a name, the volume allocation unit, whether
it's MFS or HFS, and perhaps other good things.

A small yuk: Apple's sample SCSI driver has a bug that prevents use of
the very last sector on a drive.  The bug escaped notice because volumes
are normally used only through the next-to-last sector (see above).
But, the bug serves as a signature to help you see if your disk vendor
mindlessly copied Apple's sample code, bugs and all.  Do this now: take
FEdit (or equivalent) and try to read the last sector of your disk...
Ephraim Vishniac					  ephraim@think.com
Thinking Machines Corporation / 245 First Street / Cambridge, MA 02142-1214