It's "diskConfig", camelCase.
diff --git a/openstack/compute/v2/extensions/diskconfig/requests.go b/openstack/compute/v2/extensions/diskconfig/requests.go
index e1ab48f..06a922a 100644
--- a/openstack/compute/v2/extensions/diskconfig/requests.go
+++ b/openstack/compute/v2/extensions/diskconfig/requests.go
@@ -49,7 +49,7 @@
 	base := opts.CreateOptsBuilder.ToServerCreateMap()
 
 	serverMap := base["server"].(map[string]interface{})
-	serverMap["OS-DCF:diskconfig"] = string(opts.DiskConfig)
+	serverMap["OS-DCF:diskConfig"] = string(opts.DiskConfig)
 
 	return base
 }
@@ -75,7 +75,7 @@
 	}
 
 	serverMap := base["rebuild"].(map[string]interface{})
-	serverMap["OS-DCF:diskconfig"] = string(opts.DiskConfig)
+	serverMap["OS-DCF:diskConfig"] = string(opts.DiskConfig)
 
 	return base, nil
 }
@@ -101,7 +101,7 @@
 	}
 
 	serverMap := base["resize"].(map[string]interface{})
-	serverMap["OS-DCF:diskconfig"] = string(opts.DiskConfig)
+	serverMap["OS-DCF:diskConfig"] = string(opts.DiskConfig)
 
 	return base, nil
 }