|Un mountable boot volume simply means the hard drive is not bootable. It doesn't necessarily mean the cabling is wrong. Depending on how you spliced the wires you may have crosstalk going on. IDE cables have ground wires separating the data wires to act as a shield.|
Why didn't you just replace the laptop hard drive?
You may just need to install an operating system on the 3.5" drive.
There may be other issues if the laptop originally was password protected and the hard drive was removed to circumvent a hard drive password. There may be other passwords in play.