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Question: Repair (or mark and bypass) bad sectors?

I have an old MacPro (MacPro4.1 cheese grater) running Snow Leopard 10.6.8. This machine has multiple internal 1 TB drives (as well as several external FW drives). I'll refer to the two internal drives as A and B.


A while back (several months ago), I was routinely booting off of Drive B. One day, it crashed hard, then would no longer boot. I was able to boot off another drive, and at the time I reformatted Drive A (wasn't using it for much), and then used Carbon Copy Cloner to clone the contents of (damaged) Drive B to (freshly formatted) Drive A.


There were a handful of files that could not be copied (read failures, IIRC) but nothing important (localized iTunes music files in languages I'll never use, and just a few other non-critical user data files). Nothing lost that I cared about, and apparently nothing needed for anything I use (until the day I want to play songs using a Central European language UI). So good news is that I got all the bits I needed, and I got those copied to Drive A.


I have been booting off Drive A and using it daily since then with no issues at all. Drive A seems quite healthy and reliable.


Of course, every drive seems quite healthy and reliable until the day it fails.


That day has not come, but I know it will eventually, and I'd like to set up a routine backup of the contents of Drive A, and as long as I have a working Drive B in there I'm not using for anything else, I'd like to use that for a backup.


Using Disk Utility (v.11.5.2), I've Verified and Repaired Drive B and Permissions for Drive B (several times).


For the Verify and Repair Disk operation (I've run it repeatedly), it finds no more issues except "Updating boot support partitions for the volume as required" (each time same result). I assume this is fairly trivial.


When I run Repair Permissions, it flags many (roughly 100) items where permissions differ, and then reports successfully repairing them. All are listed as being successfully repaired (which seems odd, since if I run it again, it finds the same broken permissions and again reports they're repaired). Hmmm.

There's one (only one) file that it reports it is NOT able to repair permissions on:

SUID file "System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/MacOS/ARDAg ent"

("has been modified and will not be repaired").


I would like to format this drive (Drive B), and use it as a backup volume for the current boot drive (Drive A).


I vaguely recall reading that there is a way to map out and flag bad sectors during a reformat...the idea being that the rest of the drive is probably OK, so the bad spots on the drive get marked as unusable, and the OS would then bypass them (never use them during all write operations).


Does the above make sense?


My questions:


1. Is the above really true (can bad sectors be mapped, flagged and bypassed?) If so, how does one invoke this extra magic?

2. Assuming that's a real thing and it's possible, is it just asking for trouble? It failed once already, and I consider myself lucky that I was able to pull all the data off it except for a few bits I didn't care about. 1 TB drives are not expensive - if this drive is no longer reliable enough to be useful as a backup volume, I can replace it (it's just inconvenient having to open the case and swap drives - not a huge pain in this nicely designed system, but it is one more task).


Thanks for any insights.

Mac Pro, Mac OS X (10.6)

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Mar 15, 2018 6:11 AM in response to longtimemacuser666 In response to longtimemacuser666

1. Is the above really true (can bad sectors be mapped, flagged and bypassed?) If so, how does one invoke this extra magic?

Not with any utility shipped with macOS.

Also most modern disk drive deal with mapping out bad sectors itself.


Disk Utility ONLY looks at file system metadata and the partition map. It does not actually check the health of the physical storage device.


File system metadata is the information about file ownership, permissions, time stamps, size information, where each block of the file is stored, directories, free space, etc... Basically all the stuff the file system needs to store to keep track of your files, and is not you actual file data.


2. Assuming that's a real thing and it's possible, is it just asking for trouble? It failed once already, and I consider myself lucky that I was able to pull all the data off it except for a few bits I didn't care about. 1 TB drives are not expensive - if this drive is no longer reliable enough to be useful as a backup volume, I can replace it (it's just inconvenient having to open the case and swap drives - not a huge pain in this nicely designed system, but it is one more task).

Again, the controllers on modern disks deal with all sector remapping, retries, and other error handling. The operating system is just told when there is an unrecoverable error.


Disks are not that expensive. You would be smarter to buy replacement disks and store you information on them, rather than try to extend the life of disks that have started to generate errors.


Because a disk that is starting to fail, is not going get better, and worse, that disk may get really REALLY slow as the disk controller retries and retires hundreds of times to read a marginal sector. If it succeeds after a 100 retires it will NOT tell the operating system, it will just return the data, but the operating system and you will have waited a long time for that to happen.


Get new drives, copy your data over, and discard the old drives. You will save yourself a lot of pain in the long run.


Or to put it another way. Generally the cost of a new disk drive is trivial compared to the time and effort you will put into trying to recover lost data. And if you loose family pictures, there is no amount of money that can get them back. They are one-of-a-kind.

Mar 15, 2018 6:11 AM

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Question: Repair (or mark and bypass) bad sectors?