Is it safe to use a drive that previously had bad sectors (since reallocated via a Secure Erase) My internal hard drive was playing up and diagnostics found that it had 20 bad sectors. I've since done a high security erase of the drive and all my issues seemed to go away. Disk Utility reported the drive was okay and I was able to reinstall OS X and restore from a TM backup, etc.

However, a few 3rd party apps indicate my drive now has 21 reallocated bad sectors. I imagine this is better than the previously reported 20 bad sectors (now showing as 0), but these apps are indicating my drive is still failing solely due to that. 

I haven't been able to find much information on the web about lifetime of a drive after having reallocated bad sectors, so I'm unsure of what to do. Will these reallocated bad sectors become a problem, or are they indicative that the problems haven't stopped? Should I bite the bullet and just install a new SSD?
This question could be considered 'opinion-based' however, in my opinion...

No hard drive is ever going to 'heal' over time; it can only degrade.
If it's developed bad blocks before, it will likely develop more as time goes on.

Unless you have a Time Machine constantly connected &amp; working, giving you an hourly safety-net, I'd bit the bullet &amp; swap it out now. 

Better safe than sorry.

You'll also love the speed the SSD gives you. It will feel like a new machine... or actually faster than new.