People tend to think that any type of HDD is almost the same, so they also tend to assume that any kind of free tool for general HDDs is suitable.
Laptops' HDDs are usually different from desktops' HDDs. For Laptops, there are "extra" characteristics, like for example encryption. Although the specific model ST9500420AS doesn't seem to have encryption included, the Laptop itself might have some type of encryption method.
In addition, the ST9500420AS, as other "Seagate Momentus" HDDs, are using "SmartAlign", which is Seagate's method to adapt to the new "Advanced Format" industry standard (sector size is 4096 bytes, instead of the traditional 512 bytes per sector).
If you suspect that your HDD is "dying",
get a backup/clone. Once again, I repeat,
check how your BIOS is detecting the HDD, specially its size.
After the BIOS' check, the best suggestion for your specific case would be to look for support at
http://www.seagate.com. Look for datasheets, manuals, faqs, downloads... for your
specific model. Learn about "Advanced Format" or "SmartAlign". Double check any HDD's "jumpers" position (see its updated manual and faqs for possible errata).
You could try changing the HDD to another SATA connection, if you have such, and recheck how the BIOS recognizes it (it's a long shot for your case, but who knows).
If nothing of that helps, and you still want to evaluate your HDD, then follow the instructions to download the most up-to-date
diagnostics tools from Seagate for your
specific model.
Write down any messages, codes or errors that the diagnostic tool reports, and post that info with the specific hardware configuration at Seagate's forum.
If, after all, you find out that the HDD is mechanically fine and that the BIOS is recognizing the correct size (and all the other characteristics including “
translation method”) of your HDD, then you might want to give here at UBCD's forum specific feedback so to find which tool can give you a solution to recover/backup your data.
In any case, it could be useful for other users to come back and report your results.