Disk Crashes

Disk Crashes

Author: Dave Gantz

Fixing a crashed disk, or at least recovering the data from one, can be a time-consuming process. The best protection is still the simplest one:

• keep current backups
• never treat your only copy of important data as disposable

Even so, a crashed disk does not always mean lost data. This article is about recovering what you can.

First Rule: Work from a Backup

If you are practicing these techniques, use a copy of a disk rather than the original. Recovery work can make a damaged disk worse if you are not careful.

Recommended Tools

Dave recommends having these available:

• the OS-9 Level II manual
• dEd
• a screen dump utility

The dEd build he used had a patch that recognizes and identifies the Bit Allocation Map, also called:

• BAM
• DAM

Opening a Disk in dEd

To inspect a disk directly:

OS9: DED /Dx@

where x is the drive number containing the disk you want to examine.

The trailing @ is important. It opens the disk as a raw object so that you can inspect the underlying structures directly.

Why Sector 0 Matters

The first place to examine is logical sector number 0, or LSN $00. It contains the disk identification sector and many of the values that define the disk layout.

Important fields include:

• total sectors on disk
• track size
• allocation map size
• sectors per cluster
• root directory starting sector
• disk attributes
• disk format
• sectors per track
• bootstrap location and size
• creation date
• disk name

One especially important field is the root directory pointer at offsets:

• $08
• $09
• $0A

Those bytes tell you where the root directory starts.

Why the Root Directory Matters

Once you know where the root directory begins, you can inspect the directory structure directly and start determining what still exists on the disk, what points to valid file descriptors, and what may be recoverable.

That makes the root directory the next major stop in a manual recovery session.

What This Article Tries to Teach

The original article walks through:

• how to inspect the disk descriptor sector
• how to interpret key offsets in LSN $00
• how to locate the root directory
• how to use raw disk inspection as part of data recovery

It is less about a single one-shot repair command and more about understanding the on-disk structures well enough to make careful recovery decisions.

Practical Advice

If you are dealing with a real crash:

• stop writing to the disk immediately
• work from a duplicate if possible
• gather structural information before attempting repairs
• understand the root directory and BAM before changing anything

That discipline is what gives you the best chance of recovering data instead of destroying what remains.