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You are here: MacNN Forums > Software - Troubleshooting and Discussion > macOS > Erase and reinstall OS . . .

Erase and reinstall OS . . .
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michael180
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Mar 26, 2015, 07:18 PM
 
I'm getting ready to sell my Mac Book Pro, and I want to erase it's contents, and reinstall OS X. I know there used to be some help on the Apple site, but I can't find it.

Any suggestions?

Never mind I found it . . .
     
Spheric Harlot
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Mar 26, 2015, 07:28 PM
 
Which OS?

If it's 10.7 or newer, reboot the machine holding down the "Cmd" and "R".

Once it's booted into Recovery Mode, release the keys and enter Disk Utility.

From there, Erase the main volume (usually called "Macintosh HD" or "Mac HD").

Now reinstall OS X from the main Recovery screen.
     
reader50
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Mar 26, 2015, 09:20 PM
 
If your Mac Book Pro has a hard drive (as opposed to an SSD) you should do a secure erase on the unused space. Otherwise, disk recovery tools can often find deleted files.
     
Stuke
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Mar 27, 2015, 04:12 PM
 
Why do a secure erase only on RPM hard drives? Why is it OK not to do the same, or necessary on, SSD drives? Thanks.
--
Stuke
     
Spheric Harlot
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Mar 27, 2015, 08:08 PM
 
Rotary disks have a single head per platter surface and are most effective if they can set down and just keep writing from there. SSDs don't write data in contiguous sectors - they are far more efficient if they spread data over as many sectors as they can and write those simultaneously.

Thus data erased from SSDs is for practical purposes unrecoverable, whereas hard drives can be surface-scanned and at least partially reconstructed.
     
reader50
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Mar 27, 2015, 10:24 PM
 
It depends on if TRIM is in use. Which depends on OS version, and (for early OS versions) if the user had been using a hack to enable TRIM anyway.

If TRIM is in use, freed-up blocks become undefined and would likely be silently erased by the SSD in the background. Like SH says, unrecoverable in any practical sense.

If TRIM is not in use (old MBP with 3rd party SSD, and no TRIM-enabling hack) then it will behave a lot like a rotary hard drive. Deleted files could be recovered from free space. But using secure erase could cause a significant performance hit on the SSD. If the SSD were that old, you should either replace it with a fast modern SSD, or force TRIM on somehow and reset the SSD.

So secure erase on an SSD is either unneeded, or a symptom of the SSD being used wrong, and secure erase is still not the best answer.
     
turtle777
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Mar 27, 2015, 11:29 PM
 
Or, with an old non-TRIM SSD, just fill it with junk files to the rim.

-t
     
   
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