Is it cheaper to mix drive sizes or match them in a NAS?

On raw price per terabyte the small drive often looks cheaper, but in a RAID5 or RAIDZ1 array the usable capacity is set by the smallest drive, so a smaller drive strands capacity on the larger drives you already own. As of 15 August 2026, adding three 8 TB drives beside a 24 TB drive works out to $52.37 per usable terabyte, while three more 24 TB drives cost $35.00 per usable terabyte. The cheaper sticker is the more expensive drive.

What is the real cost per usable TB when mixing drive sizes in RAID5?

In RAID5 or RAIDZ1 the usable space is (number of drives minus one) times the smallest drive, so mismatched sizes waste the extra capacity on the bigger drives. Counting the 24 TB drive already owned at about $630, adding three drives gives $52.37 per usable TB at 8 TB, $42.50 at 12 TB, $38.69 at 16 TB, $41.50 at 20 TB and $35.00 at 24 TB. The 12 TB drive has the cheapest sticker at $25.00/TB but lands third.

Does adding a small cheap drive to a big drive waste capacity?

In striped-parity layouts, yes. With three 8 TB drives beside one 24 TB drive, RAID5 gives you 3 times 8 TB equals 24 TB usable, and 16 TB of the 24 TB drive you already paid for becomes unreachable. You buy 48 TB raw and can use half of it. Unraid avoids this because it does not stripe, and Synology SHR reduces it, but ZFS RAIDZ behaves like the striped case.

Does Synology SHR or Unraid change the mixed-drive maths?

Yes, and the layout has to be named before the number. The figures here are strict RAID5 / RAIDZ1, which is the worst case. Synology SHR does better because it can use the extra space on mismatched drives when two or more drives share that capacity. Unraid does better still because it does not stripe: usable capacity is the sum of the data drives, and the only rule is that the parity drive is at least as large as the biggest data drive.

Should I buy a cheap small drive or a bigger one for my NAS?

If you run RAID5, RAIDZ1 or any striped-parity layout and already own larger drives, buy at least as large as your biggest drive, because anything smaller strands capacity you have already paid for. If you run Unraid the penalty largely disappears and the sticker price is closer to the truth. Either way, price per terabyte is still the right starting number, it just needs the usable-capacity correction applied before you compare.

Mixing Drive Sizes in a NAS: the Cheap Drive Is the Expensive One (2026)

There is a thread on r/DataHoarder from someone doing exactly the right sums on exactly the wrong number. He already owns one 24 TB Seagate Exos, he is building a four or five bay NAS, and he is weighing his next three drives:

“I was thinking of putting it in the NAS with other Exos drives (or maybe Barracudas because they are cheaper)”

PC data hoarder wanting to expand into NAS, 31 July 2026

The cheaper Barracudas are 8 TB. His Exos is 24 TB. That difference, in a parity array, is the whole story, and it flips the price ranking upside down.

I track every drive listing on Amazon.com and recompute price per terabyte daily. Every 3.5-inch drive at these capacities sits in a narrow band between about $25 and $31 per terabyte on the sticker, which is exactly why people feel safe picking on price alone. The band is real. It is also the wrong number for someone who already owns drives.

The number that actually decides it

In RAID5 or RAIDZ1, usable capacity is (number of drives minus one) times the smallest drive. Mix a small drive in with big ones and the small drive sets the ceiling for all of them. So the honest figure is not price per terabyte you buy, it is price per terabyte you can use, and it has to count the drive he already owns, because that drive’s spare capacity is what gets stranded.

Cheapest known-brand SATA at each capacity, Amazon.com, pulled 15 August 2026. He adds three drives beside his one 24 TB Exos, four drives total, RAID5.

He adds 3x	Drive sticker $/TB	He spends	Raw TB	Usable TB	Real $/usable TB
8 TB	$26.12	$627	48.0	24.0	$52.37
12 TB	$25.00 (cheapest sticker)	$900	60.0	36.0	$42.50
16 TB	$25.56	$1,227	72.0	48.0	$38.69
20 TB	$31.00	$1,860	84.0	60.0	$41.50
24 TB	$26.25	$1,890	96.0	72.0	$35.00

The real column counts all four drives, including the 24 TB he already paid for at about $630, divided by the terabytes he can actually use.

The drive with the cheapest sticker, the 12 TB at $25.00/TB, is the third best buy. The “expensive” 24 TB is the best, at $35.00 per usable terabyte against $52.37 for the cheap Barracudas. Buying the cheap drive costs him 50% more per terabyte he can actually use.

The reason is stranded capacity. With three 8 TB drives beside his 24 TB, RAID5 gives him three times 8 TB, which is 24 TB usable, and 16 TB of the drive he already owns becomes unreachable. He pays for 48 TB raw and can use half of it.

This is not carelessness, it is the right maths on the wrong number

Nobody in these threads is being sloppy. They are doing correct arithmetic on price per terabyte, which is the right north star and the number this site exists to publish. The honest follow-through is to say when it stops predicting cost, and a mismatched parity array is exactly where it does.

It is also the sequel to what capacity to buy when you are starting fresh. That one answered the question for someone buying a clean set of matching drives. This one is for the far more common case: someone who already owns a drive and is adding to it. Every thread below is that person.

How to handle mixed match HDD sizes with ZFS on a NAS?, 10 August 2026. The title is the whole question.
Upgrading storage: keeping my ST2000DM008 and adding a 4-8TB HDD, 14 July 2026. “I want to add another drive (I’ll keep using the current one) and I’m deciding between 4TB and 8TB, depending on my budget.”
Where are people finding reasonably priced drives?, 4 August 2026. The incremental buyer, in one sentence: “this is something I’ll invest more into over time, starting off with two drives and eventually adding two more (is that even feasible?)”

Name the layout first, then give the number

The table above is strict RAID5 / RAIDZ1, and that is the worst case, not the only case. A single headline figure across every layout would be wrong for most of the people asking, so the number only means something once you have said which layout it is for.

Synology SHR does better. It can use the extra space on mismatched drives when two or more drives share that capacity, so the stranding is smaller than (n-1) times the smallest drive implies. I have not modelled SHR here, and these RAID5 figures should not be read as SHR figures. If you are on SHR, treat this table as the ceiling on the penalty, not the penalty itself.
Unraid does better still, because it does not stripe. Usable capacity is the sum of the data drives, and the only rule is that the parity drive must be at least as large as the biggest data drive. On Unraid the sticker price is close to the truth and this whole article mostly does not apply to you, which is worth knowing before you change plans.
ZFS RAIDZ behaves like the table, and it also cannot easily grow a vdev, which is its own trap for the “add two more drives later” plan. If that is your setup, the number above is your number.

One more thing worth checking before you buy the cheap 8 TB drive: the cheapest 8 TB listings are frequently SMR desktop drives, which rebuild slowly in a parity array and can drop out of a resync entirely. That is a separate reason the cheap drive can be the expensive one, and it hits the exact drive class in this comparison. Check the model number against the manufacturer’s own CMR/SMR list before it goes in an array, rather than trusting a listing’s category.

The rule that falls out of it

If you run a striped-parity layout and already own larger drives, buy at least as large as your biggest drive. Anything smaller strands capacity you have already paid for, and the sticker discount does not come close to covering it. If you run Unraid, buy on sticker price and ignore most of this. Price per terabyte is still where you start. It just needs the usable-capacity correction applied before you compare, and on a mixed array that correction is large enough to reverse the answer.

You can check the current cheapest price per terabyte at every capacity on the live price-per-TB table, which is where the sticker figures above came from.