Yeah sharing HDDs with CD-ROMs on the same cable is very bad. They will conflict with each other at worst and at best they will share the channel which will seriously degrade the performance of the CD-ROM and/or hard drive.
HDDs on one cable.
CD-ROMs on another.
why would you even think of putting the HDD and CD-ROM on the same cable anyway?
Theres a reason why theres an IDE1 and IDE2 on your mobo.
I've never had a problem mixing them when i had too, but i dont because of the difference in speed. The only issue i ever had was the low reliability oif an older cd burner when on the same line as the HDD writing to it. I only set that pc up that way because the IDE2 slot was burned up.
I agree with bubba tho, issues or not you never mix them when theres an alternative.