Can I use my spare parts? Freenas Build Advice

Prebuilt NAS units are more expensive that building a FreeNAS unit from existing parts. But they will pretty much "just work" out of the box. You will spend quite a lot of time setting up FreeNAS.

You are trading simplicity and speed for $$$. If that is a good trade for you then go for it.

Your i3 will be more than sufficient for a FreeNAS unit. You are DEFINITELY better off building a FreeNAS than using USB off your router. Just make sure that the NAS is wired into the network and not WIFI.

If it is truly sleeping then you would have to send a wake on LAN (WOL) packet to wake it up. Commercial NAS devices will spin down the disks but not put the CPU to sleep. That is one reason commercial NAS devices use low power CPUs. Not much performance is required and they are powered up 24/7. You may be able to setup FreeNAS to shutdown at a specific time and wake back up at a specific time. I have always gone the commercial (Synology) route so I don't have any hands on with FreeNAS.

What is your electricity cost? $0.20 / KWh ??? It might cost an extra $1 or $2 per month, but it would take a lot of research to give you a better number....

If your electricity cost is the U.S. average of $0.115 / kWh, then each Watt the server consumes translates into about $1 in a year. So if your server averages 35 Watts, it'll cost you about $35/yr. This happy coincidence is because there are 8766 hours in a year, and 1000/8766 = 0.114.

Pretty much any CPU from Sandy Bridge or newer (i3/i5/i7 - 2xxx on up) has very low idle power draw. Obviously the newer ones are lower, but a Haswell i3-4340 should be just fine. My FreeNAS unit uses an i5-2xxx (don't remember the exact model), and with 4 drives it idles at around 35 Watts. A typical 4-drive prebuilt NAS will idle around 25 Watts, so there's really not much difference from a power perspective. The 2-drive units usually use a lower power (ARM) CPU, so those can hit 7-10 Watts (12-15 Watts with drives).

What is different is that I have a heckuva lot more power on tap than a prebuilt NAS using an ARM or Celeron processor. Mine is actually configured with a virtual machine hypervisor, and FreeNAS runs in a VM. My other VMs include a server for my business, and a "trash" Windows installation where I do anything risky, then revert to a snapshot when I'm done. That last one also handles all my video encoding tasks (the unit draws about 105 Watts at full load).

Be aware that with FreeNAS 9.x, they changed how ZFS runs. 8 GB is now the minimum, and 16 GB recommended. Before, 4 GB was sufficient (though not ideal), and you could get away with 1-2 GB at lower performance. If you don't use certain ZFS features, you can get away with less RAM (deduplication is the big memory hog). ZFS offers a lot of advantages over a regular filesystem or RAID, but the learning curve is a bit steeper. In particular, it protects against bit rot (file corruption due to changes in the disk's magnetic medium). However for this to truly be effective, the server needs to be protected against bit flips in memory as well. So the recommendation is to use ECC RAM, which more than likely your hardware doesn't support. Still, if you don't care about bit rot (you have separate backups), then this isn't an issue.

If you're going to have a lot of people hitting the file server simultaneously, you may want to consider adding a SSD to the available drives. ZFS supports caching onto SSDs (though setting it up optimally is a PITA, and it can actually slow things down if you set it up wrong). Read through the FreeNAS documentation carefully to understand the advantages and pitfalls.