Why doesn't my MacBook recognize the original HD any longer? I replaced the stock hd with a samsung ssd (840) about 18 months ago. It died last week. I tried putting the original HD back in and the computer won't recognize it. Even when I hold down the option key when booting nothing happens. When I remove the disk and connect it via USB it boots up fine as an external hard drive. Any reason why that would be?
If the original drives does work in an external case but not internally than patrix has the right idea.

We know the old drive is good as it works in an external case
We know the SSD does not work at all because it doesn't work externally or in the Mac.

In both of those cases drives are not working internally, one known bad and one known good. 

Unfortunately that points to a problem with the MacBook itself. It could be a loose connector (at the drive end or the motherboard end) And by connector I mean data and power, check both, end to end. Look for breaks, bends, crimps (that are not part of cable routing). Look for dirty or obstructed contacts. Blow some canned air into the contacts. Ifixit.com has come really great take apart instructions for just about every recent model of Mac/iPad/iPhone. Follow along with there instructions and pictures and look for differences in what theirs looks like and what yours looks like.

Logic dictates that the problem is inside your MacBook. the problem now is finding out where and then how to fix it.