diff --git a/subiquity/server/controllers/filesystem.py b/subiquity/server/controllers/filesystem.py
index 98a4e09c..026da14c 100644
--- a/subiquity/server/controllers/filesystem.py
+++ b/subiquity/server/controllers/filesystem.py
@@ -1141,6 +1141,8 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
         for disk in self.potential_boot_disks(check_boot=False):
             part_align = disk.alignment_data().part_align
             for partition in disk.partitions():
+                if partition._is_in_use:
+                    continue
                 vals = sizes.calculate_guided_resize(
                     partition.estimated_min_size,
                     partition.size,
diff --git a/subiquity/server/controllers/tests/test_filesystem.py b/subiquity/server/controllers/tests/test_filesystem.py
index e211bf6a..5f1d7626 100644
--- a/subiquity/server/controllers/tests/test_filesystem.py
+++ b/subiquity/server/controllers/tests/test_filesystem.py
@@ -902,6 +902,25 @@ class TestGuidedV2(IsolatedAsyncioTestCase):
         self.assertTrue(isinstance(resize, GuidedStorageTargetResize))
         self.assertEqual(1, len(resp.targets))
 
+    @parameterized.expand(bootloaders_and_ptables)
+    async def test_used_half_disk_mounted(self, bootloader, ptable):
+        # When a partition is already mounted, it can't be resized.
+        await self._setup(bootloader, ptable, size=100 << 30)
+        p = make_partition(
+            self.model, self.disk, preserve=True, size=50 << 30, is_in_use=True
+        )
+        self.fs_probe[p._path()] = {"ESTIMATED_MIN_SIZE": 1 << 20}
+        resp = await self.fsc.v2_guided_GET()
+
+        reformat = resp.targets.pop(0)
+        self.assertEqual(
+            GuidedStorageTargetReformat(
+                disk_id=self.disk.id, allowed=default_capabilities
+            ),
+            reformat,
+        )
+        self.assertEqual(1, len(resp.targets))
+
     @parameterized.expand(bootloaders_and_ptables)
     async def test_used_full_disk(self, bootloader, ptable):
         await self._setup(bootloader, ptable)