This drive can receive the “not formatted error” message for many reasons, such as your improper operation, virus attack, or even drive corrupted issue, etc. That’s why one should make data backups on different drives.
However, in your case, you may have no such a data backup. Therefore, to fix your problem, you need a free drive recovery tool to get your information there and format this drive. You can download this freeware from http://blog4mark.blogspot.com/2013/01/unformat-memory-c... It is really worth a shot.
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.