Better is to put a bootable backup/clone onto an ext FWHD, ensure that it boots the machine and works like the original. Then, wipe the current internal one, reformat and zero out data. and get the HD replaced. When the machine comes back or if you get it done onsite, boot the machine with the clone, erase and reformat the new HD, and restore the clone. IMO, depending on a TM backup without going through the drill of restoring it to ensure that it works is only asking for trouble.