Mountain Lion clean install
If the disk is unreadable then recovery disk will not work - and when you put in a new blank disk it won't be on that one either, so the recovery procedure outlined will not work here.

You need another boot disk. (This is where you should wish you had backups with SuperDuper! which creates fully bootable clones of your hard disk).

Assuming that you don't, you need to use your other Mac (note that the installation disks from another Mac will not work in yours - only purchased OS X disks can be used in any machine).

If you have a firewire cable the easiest solution would be to cable the two Macs together. Set one of them to TargetDisk mode, boot the other one off the good disk (I don't think it matters which is used here?).

Because your other Mac has 10.5 Leopard you don't have App Store and so can't easily get Mountain Lion (even assuming the failed machine is compatible). Instead I recommend you just clone the hard disk from machine to machine.


Download SuperDuper! (free license is sufficient, but recommend you pay for full license, it is a wise investment).
Clone the disk on the good mac to the new hard disk.
Disconnect and reboot.


You now have two working Macs with identical files on them - from there it is up to you.