FUJITSU MAT3147NC/NP - Manual (Page 46)

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Figure 3.7 Alternate block allocation by FORMAT UNIT command
During FORMAT UNIT command, alternate block allocation is conducted in following cases:
1) Unrecovered write offtrack condition during a media write
2) Uncorrectable Data Error during a media read (certification) *1
If above errors are detected during FORMAT UNIT command, the IDD allocates the alternate
block(s) to the defective data blocks. Reassign procedure itself is the same as one in REASSIGN
BLOCKS command.
*1. Certification is permitted when DCRT flag is cleared (DCRT flag=0) in FORMAT UNIT
command.
The IDD checks all initialized logical data blocks by reading them out after the above
alternate block allocation is made to initialize (format) the disk.
: Unused spare sector
: Defective sector
: n represents a logical data block number