Drive Setup: Checking For Bad Blocks

Does the Apple Drive Setup program re-map the hard disk to set aside bad or unusable sectors like a low-level format, or does it simply set up blocks and mark them all as available?
This article has been archived and is no longer updated by Apple.
Drive Setup reallocates (spares) bad blocks on all SCSI and Ultra-Wide Apple hard drives only. You must select Test Disk or initialize the drive with the write zero option selected. Selecting the low level format option however, results only in the drive rewriting the header.

Erasing the data portion of each block.

The SCSI low level format option does not verify, and thus does reallocate bad blocks. Choose low level format carefully as any difficulties encountered once the format begins can prevent use of the drive. Specifically, power failure, or otherwise interrupting the drive before format is complete may cause the drive to appear as unusable. To remedy this situation, you must run Drive Setup and initialize with the low level format option selected and allow the format process to complete. As a rule, use initialization with only the write zero option selected followed by Test Disk to achieve the best level of block data integrity.

ATA (IDE) drives internalize tracking and sparing of bad blocks. Drive Setup and ATA drivers thus have little control over this process. Which is why the low-level format option is dimmed when initializing an ATA (IDE) hard drive. Reallocation (sparing) of bad blocks that the ATA drive tracks will occur when the block is subsequently written to. Thus the best way to force the drive to reallocate bad blocks is to initialize using the write zeros feature. Use Test Disk to verify surface integrity only. Given this strategy, there on occasion may be a need to initialize using write zeros, verify using Test Disk and if any errors occur, attempt to reinitialize using write zeros again.

Selecting initialize without selecting either low level format, write zeros or both for an SCSI drive, or without selecting write zeros for an ATA drive will only write zeros to the first 1000 blocks of the drive. It will also write the partition map area including drivers (generally blocks 0 through 700 ), and in cases of Mac OS Standard or Mac OS Extended partitions, call on the operating system to initialize each volume and write the appropriate volume information.

Test Disk performs only a read-only test of all blocks. You should not rely on Test Disk to force reallocation of bad blocks on ATA drives.

Related Documents

30020 Drive Setup: Version History
21103 Troubleshooting Hard Drives: Reformatting

Published Date: Feb 20, 2012