I just had an external hard drive, formatted NTFS, seem to fail in an unusual way.

The drive seemed to spin up fine and mount fine in Linux and Windows, but appeared blank.

Well, not totally blank, it did accurately report the unused and free space, but in Nautilus/Explorer, the drive would appear blank, and trying to copy a file to it (which is a bad idea, you could overwrite recoverable data), thankfully gave an I/O error.

SeaTools reported it was fine in all of its diagnostics checks. (As it's a Seagate drive, you can reasonably suspect where I initially pointed my fingers at)

A trial version of DiskInternals' NTFS Recovery Software showed that everything seemed to be recoverable, but in order to recover, you have to buy their software. Since it listed all of the files (or seemed to according to my recollection), it seemed everything was actually okay. However, I thought the $99 was steep for my application, I would've taken the loss on the data instead.    

So, continuing my hunt for a cheaper way to recover my clearly recoverable data, I stumbled upon TestDisk, an open-source file recovery tool. It is a menu-driven application, so totally not as user-friendly as DiskInternals' tool, but I was able to figure it out without harming my data (too much*).   

I believe DiskInternals has quite a history in this market, so if you're willing to pay to recover your data, I'd just buy their software. They did make the process easy. If you're a cheapskate and don't mind further risking your data after DiskInternals has already shown that it can fix your data (I belong to the family that believes the more you mess with something, the worse your results), then TestDisk it is!

I haven't recovered everything yet (I kinda bought the external drive because I didn't have any space left...), but sampling files at random to recover yielded good results so far.

*When recovering data, be sure to copy it to another drive, otherwise you might just overwrite the data that you're trying to recover. TestDisk won't prevent you from doing so (but it didn't do so by default either, operator error???).