The drive itself is almost certainly fine. The problem is the type of drive — and it’s something manufacturers don’t advertise clearly.
Two types of hard drive — and one hidden catch
Hard drives come in two recording technologies: CMR (Conventional Magnetic Recording) and SMR (Shingled Magnetic Recording).
On a CMR drive, each track is written independently. Straightforward, predictable, consistently fast.
On an SMR drive, tracks overlap — like roof shingles. That’s where the name comes from. Overlapping tracks let manufacturers fit more data onto the same platters, which is why SMR drives are cheaper per terabyte. But there’s a significant catch.
When you need to overwrite data on an SMR drive, you can’t just rewrite one track. You have to rewrite all the overlapping tracks around it. Every write triggers a cascade of additional writes underneath. This is called write amplification, and it’s brutal on sustained workloads.
To hide it, SMR drives include a small CMR write cache. New data lands in that cache first — which is why your backup starts at full speed. But once the cache fills, the drive has to do the real SMR work: reorganizing the shingled tracks, rewriting, shuffling. That’s when it drops from 150 MB/s to 25 MB/s. And it stays there.
A backup job writing hundreds of gigabytes will fill that cache within an hour. Every time.
Why backup is the worst possible workload for an SMR drive
Most day-to-day workloads don’t expose this problem. Light file access, occasional writes, normal desktop use — the cache rarely fills. SMR drives handle that fine.
Backup is different. It writes large amounts of data continuously for an extended period. That’s exactly the workload SMR handles worst. If you’re using an external hard drive or a NAS drive as your backup destination, the recording technology matters enormously.
The frustrating part: manufacturers don’t always disclose which recording technology a drive uses. The product name often gives no clue at all.
This subtle problem even fooled our founder
Do you want to know how easy it is to fall into this trap? It caught out our company founder, Linus — the person who has spent over 20 years building backup software, who has spoken about data protection at industry conferences, and who has probably forgotten more about Windows backup than most of us will ever learn.
Linus spotted a drive on special — a Seagate BarraCuda 8TB drive to be specific. Good price per terabyte, reputable brand, nothing obviously wrong. He bought it, plugged it in as a backup destination, and noticed what you probably noticed: fast to start with, then inexplicably slow. Being a developer, his first instinct was to check whether BackupAssist had a bug.
It did not.
It wasn’t until he saw that even regular file copy suffered from this problem that he asked Claude what the problem was.
The drive was SMR.
If it can fool him, it can fool anyone. The drives don’t come with a warning label. They just look like a good deal.
What to buy instead
You want a CMR drive. CMR drives write each track independently — no write amplification, no mysterious slowdown at the one-hour mark.
Reliable CMR options:
- Seagate IronWolf / IronWolf Pro — CMR across all capacities, designed for NAS and sustained workloads. A consistent recommendation.
- WD Red Plus / WD Red Pro — CMR. Avoid plain WD Red, which uses SMR.
- Toshiba N300 — CMR, well regarded for NAS use.
- WD Gold — CMR, enterprise-grade reliability.
Drives to verify before buying:
- Plain WD Red — SMR on all capacities. Avoid for backup use.
- WD Blue — some models are CMR, some are SMR; varies by capacity and model number.
- Seagate Barracuda — SMR on standard desktop capacities. High-capacity models (20TB+) have moved to CMR, but in the ranges most commonly used for backup destinations, assume SMR unless the spec sheet says otherwise.
Before buying any drive, search the exact model number plus “SMR or CMR.” The NAS Compares website maintains a community-updated database that’s more reliable than manufacturer product pages.
One more check
If you’re using a NAS as your backup destination, check the NAS manufacturer’s compatibility list. Synology and QNAP both publish tested drive lists. A drive on that list has been validated for the kind of sustained write workload a backup job generates.
The drive is a small decision. Getting it wrong means hours of backup jobs running slow, finishing late, or quietly degrading over time — and the backup software gets the blame every time.
