The cheapest good NAS is mostly a PC you already own. This page sorts an old machine part by part: what is genuinely fine to reuse for TrueNAS, what is worth one small purchase, and the one component - tired storage drives - that decides whether the build is a success or a slow disappointment.
Before installing anything, stand in front of the old PC and make five honest decisions. The pattern of a good TrueNAS build is boring on purpose: an ordinary processor, enough RAM, a small SSD for the system, and storage drives you can trust - because the drives are the product. Everything else is furniture.
Example audit of a typical old desktop - the drives and the power supply are where judgement matters.
A NAS spends its life serving files, not gaming. Any ordinary 64-bit desktop processor and board from the last decade handles TrueNAS and family file sharing without effort. Check only three things: the board has enough SATA ports for your drives plus the boot SSD, it can run wired gigabit Ethernet, and it boots reliably. Integrated graphics is a bonus - you only need a screen during install and emergencies.
TrueNAS is built on ZFS, which uses spare memory as a fast read cache. 8 GB is the honest minimum; 16 GB is the comfortable size for a home or small-office box. Below the minimum the system will still install, but everything feels tight the moment you add apps or several users. If the old PC has free slots, second-hand RAM is the cheapest upgrade in the whole build.
This is the one place to spend. Look for drives rated for NAS use, and check the recording technology: CMR drives behave predictably in ZFS; SMR drives can slow down badly during long writes and when a mirror rebuilds after a failure. Two matched drives in a mirror beat one large drive every time. Reusing two old, unknown desktop drives is how 'free' NAS builds become expensive - a drive that was already tired does not get younger inside a NAS.
Install TrueNAS itself onto a small, cheap SSD and keep it separate from your storage pool. Older guides used USB sticks for this; they wear out silently and the failure arrives at the worst moment. A small SSD costs little, boots fast, and if it ever dies, you reinstall the system and import the pool - your data was never on it.
A NAS runs continuously, so an old, dusty power supply is the most likely sudden-death part in a reused PC - if it is ancient or noisy, replace it before the build, not after the failure. Clear the dust, confirm the fans move air over the drives, and connect the box with a network cable: a NAS on Wi-Fi disappoints everyone. A small UPS is the finishing touch where power cuts are common - it lets the NAS shut down cleanly instead of mid-write.
Every step shown live on screen - installation, pools, shares and the settings that matter.
▶️ Watch Full VideoFor learning TrueNAS, yes. As the family or office NAS, it disappoints quickly: storage hangs off USB cables, there is no room for a proper second drive, and one bump unplugs the archive. A desktop with internal drives is the honest tool for the job.
They should match in size - the mirror only uses as much of the larger drive as the smaller one offers. Same model is tidy but not required; same capacity and CMR recording are what matter.
Possibly - gaming machines idle high. Measure its idle draw with a power meter before deciding. If it sips 70 watts doing nothing, the electricity over a couple of years may cost more than an efficient small PC would. The software is free; the watts are not.