Doesn't seem like it would work in a RAID1 setup: a rebuild typically requires reading the entire source drive and writing it to the other drive in the mirror.

Doesn't seem like it would work in a RAID1 setup: a rebuild typically requires reading the entire source drive and writing it to the other drive in the mirror. Some filesystems like ZFS might understand where the used blocks are and only copy them, but others where RAID is a layer above the filesystem (Linux mdadm) will have to copy the whole drive, even if it's empty.