When a USB device isn't recognized, the cause is usually the cable, the port, a stalled driver, or a power-saving setting. Less often, the device itself has failed. The steps below start with quick checks that separate those causes, then move to fixes for Windows and Mac. The order is designed to avoid anything that could erase data.
Start with these quick checks
Do these in order. Each one narrows down where the fault is.
- Try a different USB port. Use a port directly on the computer. On a desktop, the rear ports are wired to the motherboard and are often more reliable than the front ones.
- Remove hubs, docks, and extension cables. Plug the device straight into the computer. Unpowered hubs often can't supply enough current for external hard drives.
- Swap the cable. Many cables are charge-only, and worn ones fail intermittently. Try another cable that you know carries data.
- Unplug other USB devices and restart the computer. A restart clears many temporary driver and controller faults.
- Test the device on another computer. This is the most useful check of all.
If the device works on another computer, the problem is with your computer, so continue to the Windows or Mac sections. If it fails everywhere, even with a different cable, the device is probably faulty. Skip to the last section.
Fixes for Windows
1. Check what Device Manager says
Right-click the Start button and choose Device Manager. Plug in the device and look under Universal Serial Bus controllers, Other devices, Disk drives, or Portable Devices. A yellow warning icon or an entry called "Unknown USB Device" confirms Windows detects something but can't set it up. The error text, such as "Device Descriptor Request Failed" or Code 43, usually points to a cable, port, power, or hardware problem rather than a missing driver.
2. Uninstall the device entry and let Windows rediscover it
- In Device Manager, right-click the problem entry and choose Uninstall device.
- If you see a checkbox to remove the driver software, leave it unchecked. You don't need it for this step.
- Unplug the device, restart the PC, then plug the device back in.
Windows should reinstall the driver automatically. This clears a corrupted device entry without touching your files.
3. Turn off USB power saving
Windows can cut power to USB ports to save energy, and some devices don't wake properly afterward. This is a common cause of devices that work at first and then vanish.
- In Device Manager, expand Universal Serial Bus controllers.
- Double-click each USB Root Hub or Generic USB Hub entry and open the Power Management tab.
- Uncheck Allow the computer to turn off this device to save power, then click OK. Not every entry has this tab.
You can also open Control Panel, go to Power Options, and click Change plan settings then Change advanced power settings. Under USB settings, set USB selective suspend setting to Disabled. On a laptop this may shorten battery life slightly, so you can switch it back if it makes no difference.
4. Turn off Fast Startup and do a full restart
Fast Startup saves part of the system state when you shut down, which can preserve a USB fault. Choose Restart rather than Shut down, since a restart does a full reload. To turn Fast Startup off, open Control Panel, go to Power Options, click Choose what the power buttons do, then Change settings that are currently unavailable. Uncheck Turn on fast startup and save.
5. Install Windows and chipset updates
Go to Settings > Windows Update and install pending updates, including optional driver updates if offered. If several USB ports fail on a desktop or laptop, also check your PC or motherboard maker's support page for chipset and USB controller drivers for your exact model. Download them only from the manufacturer's official site.
6. If the drive connects but doesn't appear in File Explorer
Press Windows + R, type diskmgmt.msc, and press Enter. If the drive is listed without a drive letter, right-click its partition and choose Change Drive Letter and Paths to assign one. If it shows as Unallocated or RAW, or Windows offers to format it, do not format it if you need the files. Formatting erases the drive's contents. Try another computer first, and consider data recovery software or a recovery service if the files matter.
Fixes for Mac
- Check whether the Mac sees it at all. Hold Option and click the Apple menu, choose System Information, and look under USB (or Thunderbolt/USB4 on some Macs). If the device is listed, the connection works and the problem is with mounting or permissions. If it isn't listed, suspect the cable, port, or adapter.
- Check accessory permission. In recent macOS versions, open System Settings > Privacy & Security and look for Allow accessories to connect. If it's set to ask, an approval prompt may have appeared while the Mac was locked. Unlock the Mac, reconnect the device, and approve it. This setting appears mainly on Macs with Apple silicon.
- Look in Disk Utility. Open Disk Utility, choose View > Show All Devices, and select the drive. If it's greyed out, click Mount. If mounting fails, try First Aid, which checks for file system errors. Don't choose Erase unless you've accepted losing the contents.
- Check Finder settings. If Disk Utility shows the drive mounted but it's missing from the desktop, open Finder > Settings > General and enable External disks. Also check the sidebar settings.
- Restart the Mac with the device unplugged, then reconnect it.
A drive formatted for Windows (NTFS) will usually mount on a Mac as read-only, or not usefully at all. That's a format limitation, not a fault.
Phones and other devices
If a phone isn't recognized, the cable is often the culprit, since charge-only cables are common. On Android, pull down the notification shade after connecting and change the USB mode from charging to File transfer. On an iPhone, unlock it and tap Trust when asked. Printers, cameras, and similar devices may need the manufacturer's driver or software, so check the maker's support page for your model.
How to confirm it's fixed
- The device shows up in File Explorer, Finder, Disk Utility, or the relevant app.
- Device Manager no longer shows a warning icon.
- You can open and copy a test file, and the device is still recognized after unplugging and reconnecting it or restarting the computer.
When the device itself may be failing
If the device fails on multiple computers and with different cables, the device, its connector, or (for flash drives) its storage is probably damaged. Signs include a loose connector, heat, a device that connects and disconnects repeatedly, or a drive that clicks or beeps. Repeatedly plugging in a failing drive can make recovery harder. If it holds important files, stop and contact a data recovery service rather than formatting or running repair tools. If it's under warranty, contact the manufacturer.
If the problem is limited to one computer and none of the steps above help, test a different port type and check the manufacturer's support page for your model. Persistent failures across all ports may point to a hardware fault that needs professional repair.