HDD not recognized after changing drive Letter

[windranger]

I changed the drive letter of an external 2.5' HDD (from F to D).

Then for some reason I suddenly can't use the drive anymore.

Now when I plug it, the error message is on Win7 is "You need to format the disk in drive"
The error on Win XP is D:\ is not accessible

which is quite weird.

The drive is not used much, it never had surface problems, etc.
Of course I don't want to format it because I have some info there.

Any ideas about what the problem could be...?

Thanks

 

[windranger]

Thanks for the advice. I'd go for the data recovery solution only as a last resort. There's always a possibility of erasing the data.
Besides, you can use such software only with drives that are seen by the computer.

Here's some more info:

The HDD is SATA; I can't rename it back because the computer doesn't see it, even in Disk Management.

The optical drive is E, there no drive assigned with D letter.

I tried it on a couple of different computers but same result.

Of course, any HDD can fail, but this problem coincided with changing the drive letter, so it's quite a low probability that the cause is different.