diff --git a/PVE/API2/Ceph/FS.pm b/PVE/API2/Ceph/FS.pm
index dce13e76..a76d6a56 100644
--- a/PVE/API2/Ceph/FS.pm
+++ b/PVE/API2/Ceph/FS.pm
@@ -8,6 +8,7 @@ use PVE::JSONSchema qw(get_standard_option);
 use PVE::RADOS;
 use PVE::RESTHandler;
 use PVE::RPCEnvironment;
+use PVE::Storage;
 
 use PVE::API2::Storage::Config;
 
@@ -138,6 +139,8 @@ __PACKAGE__->register_method ({
 	my $running_mds = PVE::CephTools::get_cluster_mds_state($rados);
 	die "no running Metadata Server (MDS) found!\n" if !scalar(keys %$running_mds);
 
+	PVE::Storage::assert_sid_unused($fs_name) if $param->{add_storage};
+
 	my $worker = sub {
 	    $rados = PVE::RADOS->new();