Add scratch2 convention and group write tip

Author: LinghengMeng

docs/hpc/m3.md

@@ -129,14 +129,22 @@ for d in /home/${USER}/mh42/*/; do echo "$d: $(cat ${d}STATUS 2>/dev/null || ech
 
 ## Scratch (Temporary Storage)
 
-Use scratch for large temporary files. Follow the **same folder structure** as `mh42/`:
+M3 provides two scratch filesystems — `mh42_scratch` and `mh42_scratch2`. **Use `mh42_scratch2` for all new data** — it follows the convention consistently. `mh42_scratch` has accumulated clutter from earlier usage.
+
+Follow the **same folder structure** as `mh42/`:
 
 ```bash
-mkdir -p /home/${USER}/mh42_scratch/{your_project_folder}/stack_{your_name}
+mkdir -p /home/${USER}/mh42_scratch2/{your_project_folder}/stack_{your_name}
 ```
 
 !!! warning
-    Scratch is flushed periodically. Do not rely on it for anything you need long-term — archive to S Drive or move to `mh42/` before the purge.
+    Scratch is flushed periodically. Do not rely on it for anything you need long-term — archive to [S Drive](../storage/s-drive.md) or move to `mh42/` before the purge.
+
+!!! tip
+    When creating a project folder in scratch, set group write so your supervisor and teammates can create their own stacks inside it:
+    ```bash
+    chmod g+w /home/${USER}/mh42_scratch2/{your_project_folder}/
+    ```
 
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