Ubuntu – How to fix MBR of external hard disk

That sounds like a volume flag is set. Try this procedure to let Windows assign a letter automatically.
1. Open a CMD prompt with Administrator previlege.
2. Type in DiskPart and hit Enter.
3. Run the following commands one by one. Case insensitive.

List Disk

Identify your external drive from the given list manually. Assume it's disk # (a number), and run

Select Disk #
Select Partition 1
Attribute Volume Clear NoDefaultDriveLetter

Done. Now on every Windows system it should be assigned with a letter automatically and show up in Explorer.

Practically, the disk is dead, but I've found a few things that could wring a little life off of a dead drive.

If your filesystem alone is the issue, you could run a tool that works device level, like shred in order to zero out the drive completely, and doing a reformat. I'd recommend a "full" format, and doublechecking by using a badblocks option (in linux) or chkdsk in windows with the appropriate switches - /b would work for ntfs, and /R for other filesystems.

In theory zeroing out and reformatting the drive kicks in the drive's own bad block checking and error correction, and I've kind of gotten a 'dead' drive that underwent a similar scenario running for a short time (then dying, then coming back to life, then dying again finally).

That said, you really also need to consider the health of the drive as a whole. Modern drives have a reasonable ability to self test. I'm a fan of gsmartcontrol since it highlights problem values, but I'd recommend running a short test, then if it passes a long test. If either disk health checks fail, you're better off binning the drive.

Smart Statistics are also handy - I've another answer that goes into detail on these, and if you can't get smart statistics, well the drive is also dead