I'm trying to recover some data such as movies, pictures, etc from my brother's macbook pro before he takes it in for a depot repair. Anyway, like i've been told, I boot his macbook into target disk mode and connect it to my macbook via thunderbolt cable. After I unlock his hard drive it just doesn't show in my finder window. Is there something wrong on my end? Some sort of human error? Mind you the macbook is going in to get it's motherboard, fan and hard drive replaced. I figure it's due to one of those but just incase I just thought i'd reach out.
You mention "unlocking" the drive which would suggest your brother is using filevault (the drive is encrypted). If the drive/file system is indeed damaged, I find it unlikely you will be able to recover data.
If your brother happens to be using an "old design" MacBook Pro with a non-ssd harddrive, you can still pull the drive out quite easily and connect it externally to your macbook to have a look without the motherboard being a factor. You will not loose warranty by doing this, but do consult ifixit (the best source of Apple repair guides online) to do this properly.