Diff of Infrastructure And Operations/Rathouse at 0a4629b

diff --git a/infrastructure_and_operations/rathouse.myco b/infrastructure_and_operations/rathouse.myco
index c401e6c..0e7f29c 100644
--- a/infrastructure_and_operations/rathouse.myco
+++ b/infrastructure_and_operations/rathouse.myco
@@ -12,2 +12,14 @@ frontplate number | SSD serial number
 
-use `smartctl -x /dev/sdb | grep Serial` to print serial numbers.
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+use `smartctl -x /dev/sdb | grep Serial` to print serial numbers.
+
+# mdadm recovery
+
+if a disk is pulled from a running system, mdadm assumes the worst & disconnects it from the RAID permanently.
+
+> If the system had a way of knowing that the removal and restoration was intentional, it could automatically pick it up. But a software RAID has no such knowledge, so it assumes the worst and acts as if the disk or its connection has become unreliable for some reason, until told otherwise.
+
+to re-attach a disconnected disk, do this:
+
+```
+mdadm --manage /dev/md0 --add /dev/<disk><partition>
+```
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