diff --git a/desktop/electron/services/appMode.test.ts b/desktop/electron/services/appMode.test.ts
index 1dde171a..9658da9f 100644
--- a/desktop/electron/services/appMode.test.ts
+++ b/desktop/electron/services/appMode.test.ts
@@ -117,6 +117,18 @@ describe('Electron app mode service', () => {
     })
   })
 
+  it('does not point existing config at a portable dir that cannot persist app-mode.json', () => {
+    const fakeApp = app()
+    const active = tempDir()
+    const selected = path.join(tempDir(), 'portable')
+    fs.mkdirSync(path.join(selected, 'app-mode.json'), { recursive: true })
+
+    expect(() => setAppMode(fakeApp, { mode: 'portable', portableDir: selected }, { CLAUDE_CONFIG_DIR: active }))
+      .toThrow()
+    expect(fs.existsSync(path.join(active, 'app-mode.json'))).toBe(false)
+    expect(fs.existsSync(path.join(fakeApp.getPath('userData'), 'app-mode.json'))).toBe(false)
+  })
+
   it('reports whether the default portable dir already has data', () => {
     const fakeApp = app()
     expect(detectPortableDir(fakeApp)).toEqual({
diff --git a/desktop/electron/services/appMode.ts b/desktop/electron/services/appMode.ts
index fffdb298..e02b97d8 100644
--- a/desktop/electron/services/appMode.ts
+++ b/desktop/electron/services/appMode.ts
@@ -60,6 +60,20 @@ export function writeAppModeConfig(configDir: string, config: PersistedAppModeCo
   fs.writeFileSync(path.join(configDir, APP_MODE_FILE), JSON.stringify(config, null, 2))
 }
 
+function assertWritableDataDir(configDir: string): void {
+  try {
+    fs.mkdirSync(configDir, { recursive: true })
+    const probeDir = fs.mkdtempSync(path.join(configDir, '.cc-haha-write-test-'))
+    try {
+      fs.writeFileSync(path.join(probeDir, 'probe'), '')
+    } finally {
+      fs.rmSync(probeDir, { recursive: true, force: true })
+    }
+  } catch {
+    throw new Error(`Data storage directory is not writable: ${configDir}`)
+  }
+}
+
 export function determineStartupPortableDir(
   app: AppModeAppLike,
   env: NodeJS.ProcessEnv = process.env,
@@ -129,7 +143,7 @@ export function setAppMode(
     if (fs.existsSync(selectedDir) && !fs.statSync(selectedDir).isDirectory()) {
       throw new Error(`portable config path is not a directory: ${selectedDir}`)
     }
-    fs.mkdirSync(selectedDir, { recursive: true })
+    assertWritableDataDir(selectedDir)
     targetPortableDir = selectedDir
     config = {
       mode: 'portable',
@@ -137,14 +151,15 @@ export function setAppMode(
     }
   }
 
-  writeAppModeConfig(activeConfigDir, config)
-  if (targetPortableDir && targetPortableDir !== activeConfigDir) {
-    writeAppModeConfig(targetPortableDir, config)
-  }
-
   const systemConfigDir = app.getPath('userData')
-  if (systemConfigDir !== activeConfigDir) {
-    writeAppModeConfig(systemConfigDir, config)
+  const configDirs = [
+    targetPortableDir,
+    activeConfigDir,
+    systemConfigDir,
+  ].filter((dir): dir is string => Boolean(dir))
+
+  for (const configDir of [...new Set(configDirs)]) {
+    writeAppModeConfig(configDir, config)
   }
 }
 
diff --git a/desktop/electron/services/windows.test.ts b/desktop/electron/services/windows.test.ts
index ea8fc240..daa43eef 100644
--- a/desktop/electron/services/windows.test.ts
+++ b/desktop/electron/services/windows.test.ts
@@ -1,4 +1,4 @@
-import { mkdtempSync, readFileSync, rmSync } from 'node:fs'
+import { mkdirSync, mkdtempSync, readFileSync, rmSync } from 'node:fs'
 import path from 'node:path'
 import { tmpdir } from 'node:os'
 import { describe, expect, it, vi } from 'vitest'
@@ -42,6 +42,25 @@ describe('Electron window service', () => {
     }
   })
 
+  it('does not crash when window state cannot be written', () => {
+    const tmp = mkdtempSync(path.join(tmpdir(), 'electron-window-state-unwritable-'))
+    const consoleError = vi.spyOn(console, 'error').mockImplementation(() => undefined)
+    try {
+      const app = fakeApp(path.join(tmp, 'user-data'))
+      const state = { x: 10, y: 20, width: 1280, height: 820, maximized: false }
+      mkdirSync(path.join(tmp, 'window-state.json'))
+
+      expect(() => writeWindowState(app as never, state, { CLAUDE_CONFIG_DIR: tmp })).not.toThrow()
+      expect(consoleError).toHaveBeenCalledWith(
+        expect.stringContaining('[desktop] failed to write Electron window state'),
+        expect.any(Error),
+      )
+    } finally {
+      consoleError.mockRestore()
+      rmSync(tmp, { recursive: true, force: true })
+    }
+  })
+
   it('rejects undersized or off-screen state before restore', () => {
     expect(isPersistableWindowState({ x: 0, y: 0, width: 100, height: 100, maximized: false })).toBe(false)
     expect(hasMeaningfulIntersection(
diff --git a/desktop/electron/services/windows.ts b/desktop/electron/services/windows.ts
index 4f8996fc..431873ec 100644
--- a/desktop/electron/services/windows.ts
+++ b/desktop/electron/services/windows.ts
@@ -8,6 +8,7 @@ export const DEFAULT_WINDOW_HEIGHT = 820
 export const MIN_WINDOW_WIDTH = 960
 export const MIN_WINDOW_HEIGHT = 640
 const MIN_VISIBLE_PIXELS = 80
+const failedWindowStateWritePaths = new Set<string>()
 
 export type StoredWindowState = {
   x: number
@@ -115,8 +116,16 @@ export function writeWindowState(
 ) {
   if (!isPersistableWindowState(state)) return
   const statePath = windowStatePath(app, env)
-  mkdirSync(path.dirname(statePath), { recursive: true })
-  writeFileSync(statePath, `${JSON.stringify(state, null, 2)}\n`)
+  try {
+    mkdirSync(path.dirname(statePath), { recursive: true })
+    writeFileSync(statePath, `${JSON.stringify(state, null, 2)}\n`)
+    failedWindowStateWritePaths.delete(statePath)
+  } catch (error) {
+    if (!failedWindowStateWritePaths.has(statePath)) {
+      failedWindowStateWritePaths.add(statePath)
+      console.error(`[desktop] failed to write Electron window state ${statePath}:`, error)
+    }
+  }
 }
 
 export function captureWindowState(window: BrowserWindow): StoredWindowState | null {
diff --git a/desktop/src/stores/settingsStore.test.ts b/desktop/src/stores/settingsStore.test.ts
index 452e1a52..c0fbdaa2 100644
--- a/desktop/src/stores/settingsStore.test.ts
+++ b/desktop/src/stores/settingsStore.test.ts
@@ -536,6 +536,30 @@ describe('settingsStore app mode', () => {
     })
   })
 
+  it('rolls back and surfaces app mode persistence failures', async () => {
+    const error = new Error('Data storage directory is not writable')
+    const setAppMode = vi.fn().mockRejectedValue(error)
+    installElectronAppModeHost({ set: setAppMode })
+
+    const { useSettingsStore } = await import('./settingsStore')
+    const prevAppMode = {
+      mode: 'default' as const,
+      portableDir: null,
+      defaultPortableDir: 'C:\\cc-haha\\CLAUDE_CONFIG_DIR',
+      activeConfigDir: null,
+      configDirSource: 'system' as const,
+    }
+    useSettingsStore.setState({
+      appMode: prevAppMode,
+      appModeRequiresRestart: false,
+    })
+
+    await expect(useSettingsStore.getState().setAppMode('portable', 'D:\\blocked-data'))
+      .rejects.toThrow('Data storage directory is not writable')
+    expect(useSettingsStore.getState().appMode).toEqual(prevAppMode)
+    expect(useSettingsStore.getState().appModeRequiresRestart).toBe(false)
+  })
+
   it('switches app mode back to the system data source', async () => {
     const setAppMode = vi.fn().mockResolvedValue(undefined)
     installElectronAppModeHost({ set: setAppMode })
diff --git a/desktop/src/stores/settingsStore.ts b/desktop/src/stores/settingsStore.ts
index aa40736a..634f20f0 100644
--- a/desktop/src/stores/settingsStore.ts
+++ b/desktop/src/stores/settingsStore.ts
@@ -593,8 +593,9 @@ export const useSettingsStore = create<SettingsStore>((set, get) => ({
         mode,
         portableDir: newMode.portableDir || null,
       })
-    } catch {
+    } catch (error) {
       set({ appMode: prev, appModeRequiresRestart: false })
+      throw error
     }
   },
 }))