|Inside Device Manager (Start > Run > type: devmgmt.msc), does the SD card reader have a red or yellow icon? If it's red, it may just be disabled. Open it up and set the device to enabled. If it's yellow, the device drivers may need to be reinstalled.|
If you have a drivers disc that came with the computer, check there for the proper driver. That's probably the easiest way. The second best would be to check your computer manufacturer's website for the proper driver.
Microsoft may also be able to find it online automatically (though not so well in Windows XP). Select the device in Device Manager and press the Delete key. It will be removed from the list. Open the Add Hardware Wizard (Start > Run > type: hdwwiz.cpl) and scan for new hardware.
Using the Safely Remove Hardware window is generally the preferred way to logically disconnect an external drive. That's because external drives have Drive Caching enabled by default. If you opened up the drive's properties, you could possibly find an area to disable Drive Caching. With it disabled, you wouldn't need to use Safely Remove Hardware.
Drive caching helps keep the drive from wearing out sooner because there is less data being written. The data that does get written is at a more gradual pace. Using Safely Remove Hardware ensures the data from the cache is properly written on the drive before it's ejected. If you physically eject too early, you risk data loss.
Using Safely Remove Hardware would not cause this problem. Is there a chance another person using the computer played around?
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