What to do about bad blocks on a Mac hard drive

by Brandon Yap

So you’ve used something like TechTools’s Surface Scan only to discover you have bad blocks on your Mac hard drive. Now what? Bad blocks are supposed to be automatically marked as bad by the drive controller and remapped from the drive’s factory pre-assigned reallocation pool, but sometimes this doesn’t work so you end up with bits on the drive which can’t be read from and written to properly. TechTools will not map out bad blocks for you, so what will? The answer is surprisingly obvious. Because bad blocks are reallocated on writes only, erasing the disk in Disk Utility using the Zero Out Data security option will write across every block on the disk, marking the bad blocks as it goes along so they won’t be used again.

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