We're experiencing a roughly 0.5% failure rate on encrypted drives. I believe, however, that chkdsk is largely useless on modern HDD. Because modern drives silently remap bad sectors, Windows never even knows about them -- the only time you get a bad sector reported to/by Windows is when all the hidden spare sectors have been exhausted. This is the reason why, as the previous post states, once you get a bad sector, the drive is basically finished!
Once you understand this silent sector remapping, the failure after initial encryption makes sense. Rewriting all the sectors on the drive (something that may never happen in the life of a normal hard drive) can use up all or most of the spare sectors if the drive is marginal. The problem is, of course, that no one knows (other than possibly S.M.A.R.T.) that the drive is about to fail.
For the same reason, decrypting a failing hard drive can make it worse, since it also rewrites every sector. For this reason, I attempt to use WinTech to copy files off a failing HDD whenever possible, decrypting only as a last resort.
As an aside, the SpinRite 6 recovery tool does work on an encrypted HDD since it is a sector level recovery tool that doesn't care about the actual data format. Although I've tried it on a few HDDs, they were in too bad shape to make any judgement. SpinRite may be useful, however, to test the current state of a drive. It also displays the S.M.A.R.T. data.
This is an old thread, but inbetween the last posts there was a lot of info published on my blog, and some monitoring tools.