^^ Though you can install freeNAS on the same hard drive, you have your data, but its always advisable to keep things seperate, and you dont need a complete HDD for freeNAS. It can and I think should be installed on a USB Drive. I remember I tested free NAS on the USB drive itself on my VMWare.
The below link might help ...
How to install FreeNAS to USB stick | Angel's Place
J
Just out of curiosity..... why you going for freeNAS and that too Raid 1. Just a suggestion.....go through the first page of this thread and read about Unraids Parity based security (or better google). It pretty cool. In a nut shell, you can backup up to 15 HDD on one single HDD with parity based backup ...... so you dont have to have double the space for the same. The thing is, parity based backup though can backup up to 15 disks, it can only retrieve data if any one of the HDD fails, if two fail simultaneously you
still retrieve data albeit not 100%. With Raid 1 say you have 2 data disks (and 2 for mirror i.e 4 in total), if one fails you retrieve all you data just like parity based, but if you loose both simultaneously you loose ALL your data unlike parity based security (where you still manage to save some). Raid 1 only makes sense if you have a big array 5 or more (i.e 10 HDD) ....cos its highly unlikely that all your 5 disks fail simultaneously
Another point which also should be noted is that since RAID 1 depends on mirroring the data on multiple disks it does not provide protection against data corruption due to viruses, accidental file deletions, or any other data-specific changes. coz, any such changes made are mirrored to every drive in the array segment. A virus, for example, that damages data on one drive in a RAID 1 array will damage the same data on all other drives in the array at the same time.
Please read and understand every tech intricacies for each data security technique before deciding for one ....If you wanna stick to RAID (for what ever reason) RAID 5 should be the one to go for. RAID 5 ia also the closest to to what Unraid uses for its security.