On disk failure, the storage system migrates only those disk blocks that included allocated data, and treats unallocated disk blocks as being logically zero when possible. When there is no spare disk, the source data block is logically set to zero and parity is recalculated for the RAID stripe associated with the source data block. When there is a spare, unallocated blocks on the spare are logically or physically set to zero upon migration. Write operations for the failed disk are redirected to other non-failing disks, and a record of which in-use disk blocks have been thus "migrated" to those other non-failing disks in maintained. Unused disk blocks are proactively set to zero. A target mirror copy is created using information regarding allocated disk blocks, by copying those blocks including allocated data or parity, and by clearing at the mirror those blocks not including any allocated data or parity.