diff --git a/disk/format-sdb.sh b/disk/format-sdb.sh
index b24d369077..81475c4d48 100644
--- a/disk/format-sdb.sh
+++ b/disk/format-sdb.sh
@@ -166,7 +166,11 @@ then
     check_command zfs set atime=off "$POOLNAME"
     check_command zfs set recordsize=128k "$POOLNAME"
     check_command zfs set logbias=latency "$POOLNAME"
-    check_command zfs set dnodesize=auto "$POOLNAME"
+    if [ -d /sys/firmware/efi ]
+    then
+        # dnodesize can't boot on BIOS, only UEFI mode
+        check_command zfs set dnodesize=auto "$POOLNAME"
+    fi
 
 else
     msg_box "It seems like /dev/$DEVTYPE does not exist.