A Synology DS920+ in SHR: drive 2 failed, then drive 4 at 38 per cent of the repair, and the pool marked Crashed
The owner powered down when the repair stopped. Drive 4 had unrecoverable read errors and was imaged slowly with its bad areas last; the original drive 2 was read for the stripes the repair had not reached; the healthy pair end to end. The SHR arrays were assembled from the images, the LVM volume mapped, and the Btrfs file system mounted read-only from the virtual volume. Every folder came back.
Illustrative example — replace with a genuine case