It's true that a warrant must specify two things: the place to be searched, and the items they are searching for. Without seeing the actual warrant, I can't say whether the CDs and hard drive were within the "four corners" of the warrant. But, I would be very surprised (it does sometimes happen, but not often) if the officers seized something not covered by the warrant. Am I correct that the CDs and hard drive were seized on a second search warrant--not the one initially executed to search for drugs, but one that was issued to search for evidence of his involvement in KD's disappearance? If that is the case, I would expect that the warrant specifically covered computers, hard drives, CDs etc. Again, I'd need to see the actual warrant to be sure, but that would be pretty standard. If the hard drive and CDs were seized in the drug search warrant, then it's still possible the warrant would have included digital storage devices like hard drives and CDs because they were apparently investigating the crime of maintaining a drug house and possession with intent to deliver, and they could have asked for those things with the idea that records of drug deals might be on such items. I haven't typically seen that in my experience (because drug dealers don't usually keep this info on computer, but usually on paper), but it's not impossible.
Now, search warrants can only be issued on a judicial finding of probable cause to believe that a crime was committed and that evidence of that crime is at the place to be searched. Once a warrant is issued, most states say that anything seized by an officer acting in good faith on that warrant, cannot be excluded by a judge later determining that there really wasn't enough to meet the probable cause standard. (The state I practice in is actually a little different in this regard, but that's because our state constitution contains stronger privacy rights than the 4th Amendment under the US Const.) However, if the defense can establish that any misinformation was given to the judge to obtain the search warrant or if any information was omitted from the application for the warrant, and had that misinformation not been included or omitted, there would not have been probable cause to issue the warrant, then the warrant and any evidence obtained as a result can be thrown out. (There are some other things that can lead to evidence being thrown out--if the warrant is executed in an unreasonable way, like breaking down the door when you have a key, doing the search at night w/o the warrant specifically authorizing it, failing to knock and announce your presence etc. But I haven't seen anything to indicate anything like that is in play here.)
The way Miranda rights play into this, of course, is that, if any of the information given to the judge to get the warrant was itself illegally obtained, in violation of Miranda for example, then the court has to determine if there was still probable cause to issue the warrant even without any of that information.
I hope that helps, without being too much "legalese."
Thank you AGAIN for expounding on my OTHER brief synopsis. Attorney 2. Cop 0.