Does the form factor between 3.5" and 2.5" matter in a NAS server?

You'll usually want 3.5" on anything that isn't a laptop for the price and higher max speed

And theoretical reliability. Stuff breaks down quicker at smaller sizes says my lizard brain
- Smaller stuff has smaller mass and therefore can be more reliable.
  There were portable mp3 players with mechanical hard drives that were reliable despite extreme abuse.

I think 3.5" are usually priced better per tb than 2.5" drives and performance is usually better too. So unless you feel like burning money for an inferior solution, are have some space constraints that doesn't allow 3.5" drives, I wouldn't go with 2.5" drives. They're more energy efficient though, but you'd need a fuckton of drives for that to make a worthwhile difference in your power bill.

The key here is “better performance at similar price points”. There are absolutely amazing 2.5 drives made for server applications, but they cost so much money you’re better off getting SSD these days.
Speaking of which, you should consider SSD.
- Absolutely no shot I can afford 40 TB of SSDs for my NAS
- Ssd for boot but not cost effective for nas. Nor do I trust their longevity.
Thanks, yeah i'll go with 3.5" ones then, only reason i considered it was because of some really good deals. But I'd rather stick with having a uniform set of drives. Thanks for your input!

Probably best to go with something in the 3.5" line, unless you're going enterprise 2.5" (which are entirely different birds than consumer drives)

Whatever you get for your NAS, make sure it's CMR and not SMR. SMR drives do not perform well in NAS arrays.

Many years ago I for some low cost 2.5" Barracuda for my servers only to find out years after I bought them that they were SMR and that may have been a contributing factor to them not being as fast as I expected.

TLDR: Read the datasheet

Whatever you get for your NAS, make sure it’s CMR and not SMR. SMR drives do not perform well in NAS arrays.
I just want to follow this up and stress how important it is. This isn't "oh, it kinda sucks but you can tolerate it" territory. It's actually unusable after a certain point. I inherited a Synology NAS at my current job which is used for backup storage, and my job was to figure out why it wasn't working anymore. After investigation, I found out the guy before me populated it with cheapo SMR drives, and after a certain point they just become literally unusable due to the ripple effect of rewrites inherent to shingled drives. I tried to format the array of five 6TB drives and start fresh, and it told me it would take 30 days to run whatever "optimization" process it performs after a format. After leaving it running for several days, I realized it wasn't joking. During this period, I was getting around 1MB/s throughput to the system.
Do not buy SMR drives for any parity RAID usage, ever. It is fundamentally incompatible with how parity RAID (RAID5/6, ZFS RAID-Z, etc) writes across multiple disks. SMR should only be used for write-once situations, and ideally only for cold storage.

3.5" are cheaper, go up to higher capacities (2.5" maxes out at only 5TB IIRC), and are easier to find cheap in used/refurb formats.

I wouldn't use 2.5" unless you absolutely had to for some reason.

The 2.5" drives are significantly more power efficient, often by a factor of 10. They also tend to be less noisy and produce less heat.
So in a small form factor NAS that isn't under heavy load, 2.5” drives are usually the better option.
- It looks like about 2-3W with 2.5" vs 6-8W with 3.5"
  So 3.5" drives are going to be more efficient, since you can get one that's 4x the capacity (20TB vs 5TB) for only a little over double the power usage.
  Less noise is definitely a bonus if your NAS sits next to your workstation or something though.
- Yes. Chuck some 5TB Seagate externals. They're way less pain in the ears, cooler and quieter.

2.5" disks are SMR, you don't want that in a raid.

For the record, so are a lot of 3.5s. Always read up on your drives before buying.
- Here you can check if a drive is CMR or SMR:
  https://nascompares.com/answer/list-of-wd-cmr-and-smr-hard-drives-hdd/
the 3.5" barracuda disks are SMR. the barracuda pro disks are all CMR. https://www.seagate.com/products/cmr-smr-list/
SMR ? What is that
- SMR is a relatively new disk format technology that makes drives cheaper but writes slower, which can be noticeably bad in a NAS, especially if you are using a write-intensive RAID type. Most disk manufacturers will have drives meant for NAS like WD Red or Seagate Ironwolf, and they are almost all CMR and not SMR.
- Shingled Magnetic Recording
  Basically the write head writes over part of the magnetic track below the current track, reducing the physical size of each data and increasing how much data can be stored on one side of a disk.They’re bad for random writes because the drive would need to rewrite data in the track below it as well.

Well first off, if you're building a NAS, build it out of drives that are rated for NAS use. Seagate's IronWolf line is a bit pricier than their BarraCuda but has better transfer speeds and (more importantly) better resiliency to vibration, which is important if you're putting a half dozen drives in the same enclosure and don't want them to fail prematurely.

The 2.5 unit I have runs cooler and consumes less power. It’s also more expensive.

Just buy CMR

2,5" drives are usually slower, but still about 5400rpm, which is on par with many NAS-specific 3,5" drives.

Also, you show Barracudas here, and I'd warn against them in a NAS environment. If you pick among Seagates, Ironwolf series might be what you need; otherwise, WD Reds reign supreme, just check that the specific drive you're looking for uses CMR, not SMR.

I don't think there's anything between.

lol - just realised that probably wasnt the best formulation for a question ahah

Well you're looking at it. 3.5in is faster

Generally higher storage sizes too, right? So if you want the max storage, go with 3.5"
- Cheaper too I guess

I recently started getting my drives from serverpartdeals 3.5". The refrubs seem to work great for my use case of just media. I have a second unraid server that is just 2.5" ssd's and 4 nvme's that I use for my personal files and photos since it's a much smaller and low power build I can stuff a bunch in a mini itx case so 2.5" is great for that

Depends on your NAS server. If you're like me and using an old optiplex, you can fit WAY more 2.5" drives in it, and they're pretty cheap. If you have an actual proper server chassis, then you probably want 3.5" NAS hard drives cuz warranty and all that.

Seagate has the very well earned nickname of Seabrick.

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

Fewer Letters	More Letters
CF	CloudFlare
NAS	Network-Attached Storage
RAID	Redundant Array of Independent Disks for mass storage
SATA	Serial AT Attachment interface for mass storage
SSD	Solid State Drive mass storage
ZFS	Solaris/Linux filesystem focusing on data integrity

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Power-consumption.

Also, the vibration produced by the 2.5" drives is less, but they're more-sensitive to it, to begin with.

I'd not even consider spinning-platter drives, nowadays, though:

SATA SSD's for a NAS strike me as being the sanest choice.

Samsung what are those called, Evo drives?

excellently-high MTBF, ultra-short ( compared with rotating-platters ) seek-time ( literally orders-of-magnitude quicker ), etc.

I don't know of ANY reason to go with spinning-platters, nowadays.

( & I'm saying that as a guy stupid-enough to have not realized this in time, & who spent money on such a thing, when SSD's really were the answer )

Cost? I bought 3x 8TB Ironwolf drives for £115. That'd cost about £1.5k in SSDs.
Running ZFS on consumer SSDs is absolute no go, you need datacenter-rated ones for power loss protection. Price goes brrrrt €€€€€
I too had an idea for a ssd-only pool, but I scaled it back and only use it for VMs / DBs. Everything else is on spinning rust, 2 disks in mirror with regular snapshots and off-site backup.
Now if you don't care about your data, you can just spin up whatever you want in a 120€ 2TB ssd. And then cry once it starts failing under average load.
Edit: having no power loss protection with ZFS has an enormous (negative) impact on performance and tanks your IOPS.
I don't know of ANY reason to go with spinning-platters, nowadays.
Price per terabyte is lower on HDDs. For bulk storage they are currently the best path. SSDs are catching up though, and there are cases where a SSD based NAS does make sense. But most folks at home don’t have the network capability to fully utilize their speed. Network becomes the bottleneck.