Ash, the "bad sectors don't go away" might mean a few things, but they're all going to be 'bad'.
If your prior uses of CHKDSK /F or /R listed specific sectors and then later CHKDSKs find the same values listed, this is because the drive's electronics aren't doing their job of marking off 'bad sectors' and never using them again.
Of course, if I was an HDD manufacturer and I discovered I had a batch of assembled drives that were crappy, I might not toss them out, nor would I sell them with my own label - which increases my support costs AND exposes me to bad publicity.
So, I might find an external-case maker who'd pay very little for these bad drives, sell them in his boxes and he'd take all the support costs - and any resulting bad publicity.
Mr. Samsung may have done exactly that.
Or you could have thrown it against the wall, held it over a bunsen burner too long or researched the effects of drop-kicking it out the 10th story window.
Since this has been a poor quality device for a while, I'd write it off and replace it, or understand anything saved to it may or may not be retrievable.
As for the wisdom of the next purchase - I think they're all a crap-shoot - you puts yer money down, you takes yer chances. ANY device can be bad - no matter what price or name-brand. That said, I think today's products have a better quality than yesterday's because robotic assemblies are better.
I use internals because (1) I have room in the case, (2) I don't want to have a bunch of plug-in connectors on a power-strip, and (3) because speed of data-access is so much faster.
But my externals have a portability so I can take them (along with all those bleepin' cords and plugs!! sheesh) to different locations. I accept the slower-access times in exchange for portability.
There are 3Tb External Drives that are sometimes priced very cheap, but those may be difficult (or impossible) to use on WinXP and non-64-bit OS's. I recommend limiting external drive purchases to the 2Tb models, therefore, because they're compatible with just about every other 21st Century computer.
(Back to the SAMSUNG DRIVE issue... you might find a Samsung Disk Diagnostics that will be superior to CHKDSK. I'd say it's your last hope, but there's also a chance Samsung may not have such software.)