diff --git a/src/lib_shell_services/store_errors.ml b/src/lib_shell_services/store_errors.ml
index f1fc6538f3553b21efd69b994e067590555c1b81..86fb7614076bcd66f3755e49582438999c086ed4 100644
--- a/src/lib_shell_services/store_errors.ml
+++ b/src/lib_shell_services/store_errors.ml
@@ -1296,3 +1296,40 @@ let () =
     Data_encoding.(obj1 (req "kind" corruption_kind_encoding))
     (function Corrupted_store k -> Some k | _ -> None)
     (fun k -> Corrupted_store k)
+
+(* Storage upgrade errors *)
+type error +=
+  | V_3_0_upgrade_missing_floating_block of {
+      block_hash : Block_hash.t;
+      block_level : Int32.t;
+      floating_kind : string;
+    }
+
+let () =
+  register_error_kind
+    `Permanent
+    ~id:"block_store.v_3_0_upgrade_missing_floating_block"
+    ~title:"V.3.0 upgrade missing floating block"
+    ~description:"Failed to upgrade the floating store"
+    ~pp:(fun ppf (block_hash, block_level, floating_kind) ->
+      Format.fprintf
+        ppf
+        "Failed to upgrade block %a (level %ld) for %s floating store: block \
+         not found in the index."
+        Block_hash.pp
+        block_hash
+        block_level
+        floating_kind)
+    Data_encoding.(
+      obj3
+        (req "block_hash" Block_hash.encoding)
+        (req "block_level" int32)
+        (req "floating_kind" string))
+    (function
+      | V_3_0_upgrade_missing_floating_block
+          {block_hash; block_level; floating_kind} ->
+          Some (block_hash, block_level, floating_kind)
+      | _ -> None)
+    (fun (block_hash, block_level, floating_kind) ->
+      V_3_0_upgrade_missing_floating_block
+        {block_hash; block_level; floating_kind})
diff --git a/src/lib_store/unix/block_store.ml b/src/lib_store/unix/block_store.ml
index bc93155a325d4dd0ed95f4522fe10c2c336897f6..8ce88e3cebf6bd49fffe0fdc9fd543c32e864b99 100644
--- a/src/lib_store/unix/block_store.ml
+++ b/src/lib_store/unix/block_store.ml
@@ -1739,11 +1739,31 @@ let v_3_0_upgrade chain_dir ~cleanups ~finalizers =
               (fun (block_b, _len) ->
                 let block_hash = Block_repr_unix.raw_get_block_hash block_b in
                 let block_context = Block_repr_unix.raw_get_context block_b in
-                let {
-                  Floating_block_index.Legacy.Legacy_block_info.offset;
-                  predecessors;
-                } =
-                  Floating_block_index.Legacy.find legacy_index block_hash
+                let* {
+                       Floating_block_index.Legacy.Legacy_block_info.offset;
+                       predecessors;
+                     } =
+                  try
+                    return
+                    @@ Floating_block_index.Legacy.find legacy_index block_hash
+                  with
+                  | Not_found ->
+                      let block_level =
+                        Block_repr_unix.raw_get_block_level block_b
+                      in
+                      let floating_kind =
+                        (function
+                          | Naming.RO -> "RO"
+                          | RW -> "RW"
+                          | RO_TMP -> "RO_TMP"
+                          | RW_TMP -> "RW_TMP"
+                          | Restore _ -> "Restored")
+                          kind
+                      in
+                      tzfail
+                        (V_3_0_upgrade_missing_floating_block
+                           {block_hash; block_level; floating_kind})
+                  | e -> raise e
                 in
                 let resulting_context_hash = block_context in
                 let new_value =