[RndTbl] disk cloning and bad blocks

Dan Martin ummar143 at shaw.ca
Wed Aug 19 10:59:41 CDT 2009


Further to my adventures attempting to clone the boot drive of my  
Mac ...

I attempted cloning using dd - the idea being that a block copy would  
avoid any issues with metadata that might be encountered using file  
transfers.  I/O errors were encountered (I believe a read on the  
source drive was cited).

CCC failed in block mode, and subsequently defaulted to file mode in  
spite of copying between identical types of drives.  Then it failed in  
file mode.

SuperDuper (copies in file mode only) appeared to have no difficulty -  
though the target drive would not boot until I did it a second time.   
I have since made more copies which appear to boot.  Given previous  
failures, I am not sure how much to trust that I have a faithful  
reproduction of the source drive (not that I have any other real  

I assume there is(are) bad block(s) on the source drive.  Given the  
size of modern drives (this one a Seagate 750GB), statistically I  
expect bad blocks.

In general, shouldn't bad blocks be hidden by the firmware on the  
drive?  I thought there was an internal mapping mechanism on the drive  
to exclude the use of bad blocks, which was invisible to even low  
level use such as the dd command.

Failing this, such as 'when good blocks go bad', the filesystem (HFS+  
and I presume most other modern filesystems) will catalog any known  
bad blocks to avoid using them for files.

Should I be trying to return a warrantied drive after read errors  
occur?  Discard it if it is not warrantied?

If the filesystem has isolated all bad blocks on the source drive,  
then dd conv=noerror should work so long as there are no bad blocks on  
the target drive.  Does conv=noerror pad the missing/unreadable data  
so that the ends of the source and data drive/partitions 'line up'?   
HFS+ stores important info in the last 2 sectors.

After cloning, how do you verify identical file contents between  
clones?  After a file level copy using SuperDuper, a comparison using  
FileMerge shows that many files do not match (I booted to one, so  
Spotlight data will differ).  I think FileMerge complains about  
identical identifying data between the clones that it is asked to  
compare, though there is no problem mounting identical clones.

Since I happened to be at an Apple Store in Minneapolis, I purchased  
DiskTools Pro, which is advertised to "fix bad sectors" - identifying  
which files are affected by them.  I am not sure how much to trust it,  
especially with destructive operations like defrag.  I hope to try the  
"fix bad sectors" soon.  Does anyone have experience with DiskTools Pro?

Dan Martin
GP Hospital Practitioner
Computer Scientist
ummar143 at shaw.ca
(204) 831-1746
answering machine always on

More information about the Roundtable mailing list