Hard Disk Monitoring

My D: drive has been acting slow for the last couple of days, any program accessing a file from it would slow right down and take a long time to refresh. I suspected my drive might be failing and downloaded HD Sentinel, which is reporting 5% health and just a few days before it fails. I have also run Seagate's own tools which seem to say that everything's fine, but I'm concerned that they're wrong and HDS is correct. Is it possible that this is detecting a false-positive and my problem is elsewhere, or am I going to need to backup very quickly?

No, I'm sure that there is no false alarm.
If you experience slowness and troubles you may already feel that there is something wrong - and if Hard Disk Sentinel detects and displays the problems, it confirms what you originally thought: there is a problem with the hard disk drive.

Other tools may show different situation - which is werid but very common and normal - as they do not report the real problems.
This is especially true for tools provided by manufacturers: they designed to minimise RMA requests and warranty replacements. So instead of showing, revealing and fixing problems they only designed to show less problems and issues.
In contrast, Hard Disk Sentinel designed exactly to reveal and show the real problems and possible degradations.

In most cases, you may ask warranty replacement only when the error-level threshold defined by the manufacturer reached.
Then the health in Hard Disk Sentinel would be 0%.
Until that, such tools may show "perfect" status (even on an almost failing hard disk).

Not sure about the actual situation, but generally the following topics may give additional information:
http://www.hdsentinel.com/faq.php#difference
http://www.hdsentinel.com/faq.php#health
http://www.hdsentinel.com/faq.php#tests

If you prefer, please use Report menu -> Send test report to developer option, as then it is possible to check the actual situation and advise.

Well, you have to check the data shown in the program manually. I can't recall it's name at the moment, but when I do I will post it here.

Mine says 5% too, looks like I need a new hard disk.