[olug] Odd IDE drive issues

[Eric Lusk]

I just got done recovering data from what used to be a CCTV security system for a local company.  The DVR that ran everything was apparently failing for a while, and when they tried to back up evidence of a robbery, the internal hard drive was suddenly just gone.
Their first line of support found the DVR could no longer recognize hard drives.  Not just the original hard drive, but ANY hard drive.  It could, however recognize DVD drives on the same bus, same cable, same power, you name it (3 IDE connections on the mainboard and all failed to see hard drives).
I was able to get the video back, but the drive never did show as having had a partition on it before.  I suspect the firmware that controlled this particular DVR was running Linux, although I'm not completely certain of this.  (I tried to have Linux and Windows view the partition, but none existed).
What I'm trying to figure out is:  Why couldn't the DVR see hard drives, but still see CD/DVD drives (really, this information could come in handy in future data recovery, as well as satisfying my curiosity)?
I've never seen a regular PC do any  such thing.  IDE controllers tend to be all or nothing (or just flat out flaky off/on/off if they're failing).

 I ran the Harry Potter books through a spell checker; none of those spells should work.