Doing exactly what you are suggesting, I will tell you why it's a much bigger cost than just the cost of the drive.

- The Seagate drive you mention has horrible failure rates, IIRC.

- Unless you know how to set up and monitor a ZFS pool, don't bother. Your data will not survive without this.

- Are you confident enough in your backup solution? Are your backups offsite? Are they offsite on another ZFS pool, or similar mechanism? Do you check your backups for integrity? What is your strategy for when backups (or original data) is corrupt?

- Does your home grown solution provide a secure sync capability between all your devices? Alternatively, do you have access to the photos from all your devices?

- Does your home grown solution allow your friends and family to view/download a subset of the photos from any of their devices?

- Is your solutions online a reasonable percentage of the time?

- Is your home grown solution as fast as AWS? As in, if you are traveling and want access to your data, how fast will it download/upload?

Basically, unless you plan on spending quite a bit of time setting this up, and know what you are doing, it is much much cheaper to pay from Prime, or similar.

Edit: BTW, the box you put these drives into must have ECC RAM. Without it, expect corruption. Same goes for your other box, the backup you host offsite.