BlinkenDisk

A tiny macOS utility that puts a red LED in your menu bar and lights it up whenever there's I/O activity on the local drives you choose to monitor.

It's the modern-era equivalent of the activity light on the front of an old desktop tower.

Features

• Menu bar LEDs. Lives in the menu bar ("clock bar"), not as a floating window — though it's a normal NSStatusItem, so it behaves like any other menu-bar utility. Each monitored drive gets its own LED.
• Realistic LED. Drawn as a small dome with a specular highlight; "off" is a dim tinted state so you can always see where the indicator is.
• Small status menu. Click any LED to open a menu with Settings... and Quit BlinkenDisk. Settings opens a window with a grid of local block-storage devices.
• Per-drive selection. The settings grid lets you enable each drive and choose its LED color; each LED's tooltip shows its drive name.
• Per-drive ordering. Drag rows in the settings grid to control the order of the menu-bar LEDs.
• Configurable flash duration. Default 10 ms.
• Per-drive LED color. Choose red, green, yellow, amber, or blue for each monitored drive.
• No Dock icon, no menu bar of its own. Built as an LSUIElement agent app.
• Persistent. Your drive selections, LED colors, order, and chosen duration survive restarts (stored in ~/Library/Preferences/local.blinkendisk.plist once installed).

Requirements

• macOS 12 (Monterey) or later
• Xcode 14+ command line tools (xcode-select --install) — the Swift toolchain is what builds it; you don't need the full Xcode IDE.

License

BlinkenDisk is licensed for personal, non-commercial use only. Commercial use, organizational use, and commercial redistribution require prior written permission. See LICENSE for the full terms and warranty disclaimers.

Build

From the project directory:

./build.sh

This runs swift build -c release and assembles BlinkenDisk.app. Then:

open BlinkenDisk.app

…or just double-click it in Finder. Move it to /Applications if you want it to live there. To launch on login, drag it into System Settings → General → Login Items.

To reset saved drive selections, colors, order, and duration:

open BlinkenDisk.app --args /reset

Quick run without bundling

If you just want to try it without making a .app:

swift run -c release BlinkenDisk

The accessory activation policy is set in code, so you still won't get a Dock icon — but you'll need to keep the terminal open, and Cmd-Tab may briefly show the binary on launch. The proper .app is cleaner.

How it works

• A Foundation Timer polls every 50 ms.
• For each monitored disk it reads the Statistics dictionary from the matching IOBlockStorageDriver IOKit service (cumulative Bytes (Read), Bytes (Write), Operations (Read), Operations (Write)).
• If any counter changed since the previous sample, that drive's LED is set to "on" and an off-timer is scheduled for the configured duration. Sustained I/O keeps rescheduling that timer, so the LED stays solidly lit during heavy activity and flickers briefly during small bursts.
• Drive list is built by iterating IOBlockStorageDriver services; this covers the whole disk (e.g. disk0), so all of its partitions are included by monitoring the disk once. Human-readable names come from DiskArbitration (DADiskCopyDescription).

A note on short flashes

10 ms is below one frame at 60 Hz (~16.7 ms), so the LED is programmatically on for the configured duration but the visible flash is floored by your display's refresh rate. On a 120 Hz ProMotion display the floor is ~8 ms; either way you'll always see at least one frame of red for every detected sample.

Project layout

BlinkenDisk/
├── Package.swift
├── build.sh                                  ← builds and bundles BlinkenDisk.app
├── README.md
└── Sources/BlinkenDisk/
    ├── main.swift                            ← entry point
    ├── AppDelegate.swift
    ├── StatusController.swift                ← NSStatusItem, polling
    ├── SettingsWindowController.swift        ← settings dialog
    ├── DiskMonitor.swift                     ← IOKit + DiskArbitration
    └── LEDRenderer.swift                     ← draws the LED

Limitations / things you might want to add

• Whole disks only (e.g. disk0), not individual partitions. The IOKit statistics are exposed at the IOBlockStorageDriver level which sits above the partition map, so this is the natural granularity. Per-volume monitoring would need a different data source (e.g. parsing iostat or fs_usage).
• No separate read/write indication. Easy to add: render two LEDs (green for read, red for write) by comparing bytesRead and bytesWritten deltas separately in StatusController.poll().
• Network volumes don't show up — they're not block-storage drivers.