diff --git a/CHANGELOG.md b/CHANGELOG.md
index 747c813..c082320 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -56,6 +56,9 @@ per the process in [`docs/releasing.md`](docs/releasing.md).
 - `pithead doctor` now checks that Docker is enabled to start at boot (systemd) and warns if not —
   `restart: unless-stopped` only brings the stack back after a reboot when the daemon does too,
   which matters for an unattended miner (#137).
+- Low-disk warning badge in the dashboard header (#138): a heads-up at 85% used of the data
+  filesystem and a prominent critical alert at 95%, on both the sync and main screens — the disk
+  bar alone is easy to miss, and a full data disk corrupts the Monero database mid-write.
 
 ### Changed
 
diff --git a/build/dashboard/mining_dashboard/config/config.py b/build/dashboard/mining_dashboard/config/config.py
index d4f8093..7aa4c3a 100644
--- a/build/dashboard/mining_dashboard/config/config.py
+++ b/build/dashboard/mining_dashboard/config/config.py
@@ -9,6 +9,13 @@
 DISK_PATH = '/data'
 DB_FILE_PATH = os.path.join(DISK_PATH, "mining_data.db")
 
+# Low-disk warning thresholds (% used of the data filesystem), surfaced as a top-bar badge (#138):
+# a heads-up at WARN, a prominent critical alert near full. The chains keep growing and a full data
+# disk corrupts monerod's DB mid-write, so this is a runtime guard distinct from the setup-time
+# pre-flight estimate. (The disk *bar* uses its own colour thresholds; these gate the badge.)
+DISK_WARN_PERCENT = 85
+DISK_CRITICAL_PERCENT = 95
+
 # --- Data Source File Paths ---
 # File paths for JSON metrics generated by local collectors
 STRATUM_STATS_PATH = f"{BASE_STATS_DIR}/local/stratum"
diff --git a/build/dashboard/mining_dashboard/web/views.py b/build/dashboard/mining_dashboard/web/views.py
index 219a4d4..871c323 100644
--- a/build/dashboard/mining_dashboard/web/views.py
+++ b/build/dashboard/mining_dashboard/web/views.py
@@ -15,7 +15,7 @@
 import bisect
 import logging
 
-from mining_dashboard.config.config import HOST_IP, UPDATE_INTERVAL
+from mining_dashboard.config.config import HOST_IP, UPDATE_INTERVAL, DISK_WARN_PERCENT, DISK_CRITICAL_PERCENT
 from mining_dashboard.helper.utils import (
     format_hashrate, format_duration, format_time_abs, is_ip_address, detect_host_ipv4,
 )
@@ -567,6 +567,17 @@ def build_badges(data, metrics, mode_variant):
     elif metrics.monero_mode == "Full":
         badges.append({"text": "XMR Full", "variant": "outline", "title": "Monero blockchain is full (not pruned)"})
 
+    # Low-disk badge (Issue #138). The data filesystem fills as the chains grow and logs accumulate;
+    # a full disk corrupts monerod's DB mid-write. The disk *bar* shows the percentage, but it's easy
+    # to miss — surface a prominent top-bar badge near full, on both the sync and main screens.
+    disk_percent = (data.get('system', {}).get('disk', {}) or {}).get('percent', 0) or 0
+    if disk_percent >= DISK_CRITICAL_PERCENT:
+        badges.append({"text": f"⚠ Disk {disk_percent:.0f}% full", "variant": "bad",
+                       "title": "The data disk is almost full — free space now; a full disk can corrupt the Monero database."})
+    elif disk_percent >= DISK_WARN_PERCENT:
+        badges.append({"text": f"Disk {disk_percent:.0f}% full", "variant": "warn",
+                       "title": "The data disk is filling up — free space or move a data_dir before it runs out."})
+
     return badges
 
 
diff --git a/build/dashboard/tests/web/test_views.py b/build/dashboard/tests/web/test_views.py
index edb1ccc..28815de 100644
--- a/build/dashboard/tests/web/test_views.py
+++ b/build/dashboard/tests/web/test_views.py
@@ -371,6 +371,23 @@ def test_no_prune_badge_when_unknown(self):
         out = build_badges({}, _metrics(monero_mode="Unknown"), "ok")
         assert not any("XMR" in b["text"] for b in out)
 
+    def test_disk_badge_critical(self):
+        out = build_badges({"system": {"disk": {"percent": 96}}}, _metrics(), "ok")
+        assert any(b["variant"] == "bad" and "Disk 96% full" in b["text"] for b in out)
+
+    def test_disk_badge_warn(self):
+        out = build_badges({"system": {"disk": {"percent": 88}}}, _metrics(), "ok")
+        assert any(b["variant"] == "warn" and "Disk 88% full" in b["text"] for b in out)
+
+    def test_no_disk_badge_when_ample(self):
+        out = build_badges({"system": {"disk": {"percent": 50}}}, _metrics(), "ok")
+        assert not any("Disk" in b["text"] for b in out)
+
+    def test_no_disk_badge_when_missing(self):
+        # No system/disk data (e.g. an early poll) must not emit a spurious or crashing badge.
+        out = build_badges({}, _metrics(), "ok")
+        assert not any("Disk" in b["text"] for b in out)
+
 
 # --- System (presentation thresholds) -------------------------------------------------