Please bear with me as I go through a somewhat complex set of troubleshooting and the eventual results ...
My Hughes SD-DVR40 (with WD Caviar Blue 250gb replacement [IDE] hard drive) started having problems in the spring -- many recorded programs would play with almost unwatchable pixelation, pauses, etc. The unit would also periodically reboot (but always come back up). Then about two weeks ago it failed to boot (stuck at the "Powering Up" screen). I figured the hard drive (which replaced the original 40gb drive in 2011) was bad. I had a spare WD10EARS 1tb SATA drive that I I had not been using and decided to try that. Ordered a cheap SATA to IDE adapter ($6.95) from Amazon. Then I used dd_rescue to copy the 250gb drive to the 1tb SATA drive (under linux). Strange that I got no errors. I also ran the Western Digital DOS-based diagnostics (from a USB thumb drive) on the supposedly bad disk and got zero errors on both the short and long tests. Hmmm. Also used the WD program to turn off idle parking (wdidle3) on the WD10EARS 1tb drive.
Next my problems compounded after putting the 1tb drive in the dvr40 when it would not even display the "Powering Up" screen and no front panel lights at all. Thinking that maybe something was wrong with the SATA/IDE adapter I put the original IDE 250gb drive back in. Same problem -- no power. Sure looked like a power supply issue now. So I went on ebay and bought a used dvr40 and then swapped in the power supply from the ebay dvr40. Since the original 250gb drive had no errors on the WD diagnostic tests I left it in. Now the dvr40 powered on and went to the "Powering Up" screen (green light lit), but did not get past that. Oh well. Next step: put in the 1tb drive (with SATA/IDE adapter) and tried that. Bingo! Worked perfectly -- all my recorded programs and season passes still there. The nice thing about the $6.95 SATA/IDE adapter is that it has a drive activity LED that you can watch with the dvr cover off -- it pretty much is constantly flashing (even when no programs are being recorded). No wonder Tivo disk drives fail so frequently -- they are active 24 by 7.
So my question: how is it possible that a disk drive can pass all diagnostic tests but still fail to boot? The obvious answer would be corruption on the disk. But in my case we can rule that out since I had used dd_rescue to make an exact block by block copy of the original drive. I'm stumped.