Choosing between a 4TB NAS hard drive and an 8TB NAS hard drive comes down to bays, budget and how much risk you accept during a rebuild. Neither size is always better. This guide compares them on cost per terabyte, rebuild time and capacity, and ends with a simple way to decide.
Capacity: how many bays does each size need?
Bigger drives reach the same usable space with fewer bays. Here is what it takes to get 16 TB of usable space at different protection levels.
| Target: 16 TB usable | With 4 TB drives | With 8 TB drives |
|---|---|---|
| RAID 5 | 5 drives (5 bays) | 3 drives (3 bays) |
| RAID 6 | 6 drives (6 bays) | 4 drives (4 bays) |
| RAID 10 | 8 drives (8 bays) | 4 drives (4 bays) |
If your NAS has two or four bays, 8 TB drives let you reach a given capacity without buying a bigger enclosure. If you already own a larger NAS with free bays, 4 TB drives can be a flexible way to grow. Check the maths for your own setup with the RAID calculator.
Cost per terabyte
Larger drives often cost less per terabyte, but not always. The best value size changes over time and between brands, so compare the price per TB on the day you buy rather than trusting a rule. Also remember that fewer, larger drives mean fewer bays used, which can save money on the enclosure and on power.
- Divide the price by the capacity for each candidate drive.
- Add the number of drives you need for your RAID level.
- Compare total spend for the same usable capacity, not the price of a single drive.
Rebuild time and risk
When a drive fails, the array rebuilds onto a replacement by rewriting the whole drive. An 8 TB drive has twice as much to write as a 4 TB drive, so a rebuild generally takes longer. The exact time depends on the NAS, the RAID level and how busy the array is, so treat any quoted figure with caution.
During a rebuild your array has less protection. With RAID 5 and large drives, that is a real consideration. This is why many people pick RAID 6 or SHR-2 for 8 TB drives and above. See RAID explained for the trade-offs.
CMR and NAS ratings matter more than size
- Choose a drive marketed for NAS use, designed for continuous operation and multi-drive vibration.
- Prefer CMR recording. Shingled (SMR) drives can slow down badly during long writes and rebuilds.
- Check the datasheet for the workload rating and warranty length.
- Avoid mixing batches: buy drives at different times or from different shops if you can.
Our guide to choosing NAS hard drives goes through each specification in detail.
Which size should you buy?
| Your situation | Better fit |
|---|---|
| 2-bay NAS, want room to grow | 8 TB (or larger) |
| 4-bay NAS, moderate data, tight budget | 4 TB drives, with a plan to upgrade later |
| Lots of video, 4 or more bays | 8 TB drives in RAID 6 or SHR-2 |
| Mostly documents and photos | 4 TB is often plenty |
| Already own spare bays | Either, whichever has the better price per TB |
Power, noise and heat
Fewer drives generally means less power draw, less noise and less heat in the enclosure, which matters if the NAS sits in a living space. Two 8 TB drives will usually run cooler than four 4 TB drives holding the same data. Check the datasheet for power figures and noise ratings on the specific models you are considering, because they vary between product lines.
Upgrading later
Starting with 4 TB drives and replacing them with 8 TB drives later is a common path. It works, but you pay for every drive twice and rebuild once per replaced drive. Starting with larger drives costs more today and saves effort later. If your NAS supports adding drives to an existing array, you can also start with fewer drives than bays and add more as your library grows. Check that your model supports expanding with the RAID level you choose.
4TB vs 8TB NAS hard drive FAQ
Is an 8TB NAS hard drive better than a 4TB one?
Not automatically. It gives more capacity per bay and is often better value per terabyte, but rebuilds take longer, and one drive holds more data at risk.
How many 4TB drives equal one 8TB drive?
Two 4TB drives hold the same raw capacity as one 8TB drive, but they use two bays and, in RAID, lose space to parity or mirroring.
Can I mix 4TB and 8TB drives in a NAS?
You can, but standard RAID treats all drives as the smallest size. Systems with flexible RAID, such as Synology SHR, use mixed sizes more efficiently.
Do I need NAS-specific drives?
They are strongly recommended. NAS-rated CMR drives are built for continuous use and array behaviour.
