The Windows operating system recognizes and automatically installs most portable hard drives as soon as you connect them. This wasn't the case with older operating systems, as they often experienced problems with missing drivers; however, an unrecognized external hard drive on Windows XP and later versions of the operating system most often occurs due to loose wiring, hardware faults, an unpartitioned drive or a drive using an incompatible file system.
Check Your Connections
A loose cable can cause an external drive to not show up on your system or to show up but fail to open. Try plugging the drive into a different USB or eSATA port, or switching to an alternate cable if you have an extra one around. If you placed the drive in an external closure yourself, open it up and make sure you attached the data cable firmly -- a loose data cable can keep the drive from working even if it still turns on.
Partition and Format New Drives
A blank external hard drive will not appear alongside your existing drives until you partition it. Some off-the-shelf drives come partitioned, but if your new drive doesn't show up, check the Computer Management tool, which you can find by searching on the Start menu or Start screen. Look for your new drive in the lower panel of the Disk Management section. If you see a large block marked "Unallocated" or "Free space," right-click it and pick "New Simple Volume" from the context menu. Proceed through the setup -- leaving all settings on their defaults unless you need to change them -- to partition and format the disk.
Every drive uses a file system to store data. Windows can read several file system types, but not all. All newer versions of Windows use NTFS by default, but if you connect an NTFS drive to a pre-XP or non-Windows computer, it may not recognize it. Similarly, if you connect a drive using a non-Windows file system, such as Mac OS X's HFS, even a new version of Windows won't open it. The simplest solution is to send files between incompatible systems using another method, such as email, but you can also download tools to read some types of drives (see Resources).
If no other solutions fix your drive issue, the drive itself may have failed. If this occurs, the best way to recover lost data is by paying for professional disk recovery. Before that last resort, however, try using the drive on another computer, in case your PC's ports are faulty. The drive enclosure itself may also have broken, so take the drive out and place it in another enclosure, or install it internally in your computer.
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