I used to have 3 HD's the master which was a 80gig IDE, 1st slave which was a 40gig IDE and 2nd slave which was a 300gig sata. All was sweet and ran just fine. Then I upgraded and purchased a second Sata which is 500gig. I transfered all from my 300 to my 500gig then reformated my 300gig and copied my 80gig to it. Now I have set up my 300gig sata as my main with the 500gig as my first slave and I wanted use one of the ide's as my second slave but the pc won't read either of them ( the 40 or 80 ) Why is this happening? My PC won't read the IDE HD's any where not even in the bios.
Would it read it if I was to make one of them an external drive? and if so what will I need to do that?

Checked the obvious like the secure cable, the IDE controller's on in the BIOS, both drives have power and one's master and one's slave?

Yes I have checked and double checked all the above.
So you can run Sata as the main and first slave and have the second slave as an IDE?

There is no concept of Master and Slave when using SATA drives. Every drive is a Master.
If the board has the capabilities, you can fill up every SATA port and every IDE port with a drive and have all six going. Not a problem.