Computer does not see an external hard drive on Windows 7

My external hard drive does not show up when I plug it into a Windows 7 machine, no drive letter in Computer, though the drive powers on.

What is the diagnostic order for a drive that is there physically but invisible?

Powers on but no drive letter is a specific and diagnosable state, since the drive being physically present means the fault is somewhere in the recognition chain, checked from the outside in:

1

Rule out the cable and port first, the cheapest causes: try a different USB cable and a different port, ideally a rear port on a desktop for full power, since a failing cable or an underpowered port produces exactly this, a drive that spins but does not enumerate. A different computer entirely is the fastest isolation, working there points at this PC, failing everywhere points at the drive.

2

Check Disk Management, where an invisible drive often actually appears: right click Computer, Manage, Disk Management and look for the drive there even without a letter in Computer. This is the key diagnostic, since a drive showing in Disk Management is recognized by Windows and just needs attention, while a drive absent from Disk Management too points deeper at drivers or hardware.

3

Act on what Disk Management shows: a drive there with a healthy partition but no letter needs a letter assigned, right click the partition, Change Drive Letter and Paths, Add. A drive showing as Unallocated or RAW has a partition problem, recoverable but do not initialize or format if the data matters, since that erases it, the trap Disk Management's prompts lead people into. A drive showing but Offline right clicks to Online.

4

Check Device Manager for driver issues when Disk Management does not show it: Device Manager, look under Disk drives and USB controllers for the device with a warning icon and update or uninstall and reconnect to reinstall the driver. A yellow warning here explains an invisible drive, and a drive appearing in neither Disk Management nor Device Manager after cable and port checks points at drive hardware failure.

The data safety warning that matters most: when Disk Management shows the drive as RAW, unallocated or prompts to initialize or format and the drive holds data you need, do not accept those prompts, since initializing or formatting to make the drive usable erases the data recovery would otherwise reach. A drive that became RAW or unreadable but physically works is a data recovery situation, run recovery tools against it before any format, the difference between recovering the data and destroying it while trying to access it.

Disk Management showed it with no letter, assigned one and it appeared instantly, such a simple fix for something that looked like a dead drive. The do not initialize a RAW drive warning is filed for future reference, I can see how easily that trap catches people. Cable was fine, just the missing letter.