This is from memory so sorry if there is something new to do it but,
1. The 2.5" IDE cable is not available. This is the cable tha runs from the Bay to the MOBO, not the interchangable pin out at the back of the HDD?
2. If this is the 2.5" IDE cable then go to a laptop store to buy a new one, there are cheap ones or proprietry gear from HP if required, the latter will be most probable.
3. If its not going to carry an internal HDD then you need a Bios that is able to BOOT from external device, or boot via USB at the least,remember they all have boot from NIC so if you want that is still an option of the server is set up to allow wake on lan kind of stuff. The Bios thou is where you will need to set the boot order with 1. Boot with external device then 2. FDD or a CD option and so on. The nic is complicated and restricts the laptop to RJ45 or a wireless radius.
There is no other way to get the laptop to recognise an external device unless you pin the MOBO correctley , I.E. Dell has specific stuff to allow 2.5" stuff to work in a workstation (GX270.etc...) so the cable to allow a 3.5" HDD to connect to a laptop MOBO may have to be spliced yourself with the schematics of both your pin in MOBO and pinout HDD. That would be then run through the opening where the drive use to sit and out to an external HDD but you would also have to splice the power via a normal PSU or other device to give the correct ac/dc conversion.