[hfs-user] Wrapper and Readme

Mark Day mday@apple.com
Fri, 3 May 2002 09:48:52 -0700

On Friday, May 3, 2002, at 08:55 AM, Entwicklung wrote:

> Btw. Mark had written sth about Disk First Aid and the alternate volume
> header - one problem I've been having right from the start is that Disk
> First Aid is unable to read my CD and Norton Disk Doctor detects 'major
> problems' on my CD. It seems as though this is due to the runout-blocks
> being  appended to the end of my image i.e. after the sector in which the
> alternate volume header is present. This doesn't really disturb me too much
> since the Mac mounts my CD's and reads them without a problem but I'd like
> to know if there's sth I can do to overcome this.

Remember that the alternate MDB or alternate Volume Header starts 1024 bytes from the end of the volume.  So, on a CD with 2048-byte physical sectors, it would start in the middle of the last physical sector.  In the case of a wrapped HFS Plus volume, the alternate MDB is 1024 bytes from the end of the volume, and the alternate Volume Header is 1024 bytes from the end of the embedded part of the volume.

The last 512 bytes of the volume (and embedded volume) are reserved for uses outside the filesystem.  Apple's manufacturing process uses the space for various things, without having to disturb the actual volume contents.

I don't know much about the low-level data organization on CDs, but I would think that the driver shouldn't include runout blocks as part of the data (i.e. part of the logical device or partition).  Might you actually be writing data past the end of your volume?

If you're working with disk images, you could try truncating the image to 1024 bytes past the start of the alternate MDB (or alternate Volume Header for unwrapped HFS Plus volumes).  But you still ought to figure out why there is extra data past the logical end of your volume (or why you aren't putting the alternate MDB/VH in the correct spot).

Disk First Aid tries to use the alternate MDB/VH to identify the volume and find the various structures.  The idea is that if the first few blocks of the volume have been overwritten, Disk First Aid can find and fix the volume using the alternate structures, and write corrected information back out to the main MDB/VH.  Current versions of DFA should try to find the main MDB/VH if it can't find any alternate.  Norton Disk Doctor apparently only uses the main MDB/VH to find the volume's structures, and then later compares the alternate to the main copy.