+// PutBlock stores the given block on one or more volumes.
+//
+// The MD5 checksum of the block must match the given hash.
+//
+// The block is written to each writable volume (ordered by priority
+// and then UUID, see volume.go) until at least one replica has been
+// stored in each of the requested storage classes.
+//
+// The returned error, if any, is a KeepError with one of the
+// following codes:
+//
+// 500 Collision