BLU V0.90 bug - update checksums error/warning

From: James MacPhail <gg__at_email.domain.hidden>
Date: Thu, 20 Feb 2014 14:40:36 -0800

If you use BLU, take note:

BLU has an option to "Set Writes to Update Checksums" when writing to a parallel port hard disk. In the current version of BLU, this changes the last byte of the sector tags so that the xor of all the bytes in the data and tags is zero.

However, it now appears that this is applicable to MacWorks environments only. The Programmers Workshop and Lisa Operating System environments use a different byte in the tags for the xor checksum, and so (if a change is necessary) the BLU function will change the wrong byte of the tags.

The PW & LOS tags contain file system information (such as forward/backward links to consecutive sectors in a file) that appears to be intended for possible disk scavenging/repairing, so it is not necessarily a problem if the tag bytes are mangled, but probably it is better to not do so casually.

Regardless, BLU may be used to verify the sector checksums. In the event that a checksum error prevents a sector from being read, using BLU to update (the wrong byte of) the tags still may make the data recoverable (at the cost of making the tag bytes incorrect).

Another option would be to transfer the hard disk image to another computer, edit the correct tag byte of the problem sector, and write it back to the hard disk.

If you plan to use the checksum update function, it would be wise to make a backup image of the hard disk first.

A future version of BLU may have the option to update the (different) checksum tag byte used by PW & LOS.

In general, it appears that non-Apple environments such as Xenix do not implement a sector checksum; the tags are unused/spare and changing them is neither necessary nor problematic.

The BLU web page is at: http://www.SigmaSevenSystems.com/BLU

James MacPhail
Sigma Seven Systems Ltd.

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