diff --git a/templates-updates.fif.json b/templates-updates.fif.json
index ca250eca..4f40e646 100644
--- a/templates-updates.fif.json
+++ b/templates-updates.fif.json
@@ -287,7 +287,8 @@
                 "BOOTFROM": "c",
                 "GRUB_POSTINSTALL": "selinux=0",
                 "HDD_1": "disk_f%VERSION%_minimal_3_%ARCH%.img",
-                "NUMDISKS": "2",
+                "HDDSIZEGB_3": "25",
+                "NUMDISKS": "3",
                 "POSTINSTALL": "_live_build",
                 "ROOT_PASSWORD": "weakpassword",
                 "USER_LOGIN": "false"
diff --git a/tests/_live_build.pm b/tests/_live_build.pm
index 9d224bd9..5b6285ba 100644
--- a/tests/_live_build.pm
+++ b/tests/_live_build.pm
@@ -10,6 +10,16 @@ sub run {
     my $arch = get_var("ARCH");
     my $subv = get_var("SUBVARIANT");
     my $lcsubv = lc($subv);
+    if (get_var("NUMDISKS") > 2) {
+        # put /var/lib/mock on the third disk, so we don't run out of
+        # space on the main disk. The second disk will have already
+        # been claimed for the update repo.
+        assert_script_run "echo 'type=83' | sfdisk /dev/vdc";
+        assert_script_run "mkfs.ext4 /dev/vdc1";
+        assert_script_run "echo '/dev/vdc1 /var/lib/mock ext4 defaults 1 2' >> /etc/fstab";
+        assert_script_run "mkdir -p /var/lib/mock";
+        assert_script_run "mount /var/lib/mock";
+    }
     # install the tools we need
     assert_script_run "dnf -y install mock git pykickstart tar", 120;
     # base mock config on original