External Hard Drive Red Green No letter Recognition

One day my external drive stopped being recognized (500 GB Western Digital). Initial error was I/O input output error.It does show up in Device Manager as "usb mass storage device". I tried to reinstall driver and it says it is the same driver . I do not think it is a physical problem with the hard drive as when it is not plugged in to the usb port the green light is constant, yet when it is plugged in it switches from red to green intermittenly. I did think it could have been because of the case so I bought another external sata casing but the same problem still occurs.

When I try to boot up with the computer off the computer will hang.

I have 150 gbs of important data so would prefer to not reformat .

Any suggestions ?

Thanks !

Yes to 2 other computers.


I posted this later:
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So I have a 500gb External Western Digital drive that all of a sudden stopped being recognized.It had (hopefully still has 150 gb of data on it).


I went through several steps and now device manager in XP recognizes it as a healthy 465 gb unformatted drive.


So I go to DOS and enter E: so I can go and run chkdsk /r and then it hangs forever and finally and I finally get a cyclic redundancy error.

I have connected it to two computers and the same error occurs.

Any other diagnostic tools or other suggestions ?
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Thanks !