Re: Low level SCSI fixer

[dempson@swell.actrix.gen.nz (David Empson)]

In article <CL4F3o.D8F@mail.on-line.co.uk>
tombo@mail.on-line.co.uk (Richard King) writes:
> I'm still looking for a Low Level SCSI editor, to try and recover several
> partitions which got 'lost' when my partition map got zapped. Anyone have
> any suggestions about where I might find a program to assist me in this
> (possibly futile) enterprise? Apart from - as one respondent put it -
> 'wrolling (sic) my own'.

Someone else suggested Chinook SCSI Utilities as a possibility.  This
program requires an Apple SCSI card.

If you knew the exact sizes and positions (to the physical block
number!) of the original partitions, it would be possible to re-create
the partition map using CSU.  If you didn't re-format the newly created
partitions, then the old data (directory and all) should still be there.  

This is a special feature of CSU - other partitioning programs will
wipe at least the first few blocks of each partition, or will insist on
formatting the partitionts.  CSU lets you re-partition part of the drive
without affecting lower-numbered partitions (you have to remember to
turn off the format option for each partition you want to preserve).


This technique will be difficult to use unless the drive was
originally partitioned using CSU.  Other partitioning programs may
create special partitions, which CSU cannot re-create, or may use a
different size for the partition map.
 
For example, Advanced SCSI Utilities creates four partitions for
Macintosh drivers, which CSU cannot create.  You would have to allow
for these by setting up a single partition to cover this area.  This
will usually end up being the first partition on the drive, which
would interfere with booting the drive.

If you don't know the exact partition positions and sizes, you could
make an educated guess, then use a block editor to examine the data on
the disk to see how far out you are.  Re-partition to the correct
size, and you should be away.

Note: if CSU's partition map is larger than the original one, you would
not be able to recover any data from the first partition on the drive,
since the partition map would overwrite part of the first partition,
and all of the block numbers in the directory and file structures
would be wrong.  CSU creates a fixed-size partition map.


If you did succeed in getting the data back again by using any of
these techniques, I'd strongly suggest copying the data off the hard
drive, re-partitioning it properly (if you had to use any special
tricks), then copying the data back on again.


The problems with CSU:

- it only supports seven partitions
- created partitions cannot exceed 32 megabytes
- it requires an Apple SCSI card (either type).

Oh how I wish for an updated version...  Maybe I should write my own
partitioning software...  :-)


Any so-called "low-level SCSI editor" would have to access the drive
at the physical block level.  This requires direct SCSI calls,
bypassing the partitioning system.  It is reasonably easy to do this
on an Apple High-Speed SCSI card.  It is considerably harder on the
original Apple SCSI card.  I don't know about a RamFAST.
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand