Using a QLC SSD for Backups. Am I Insane?

I have a properly working Seagate Backup Plus Hub. However, I’m now using a Samsung 870 QVO for Time Machine backups on my Mac, despite its bottom-of-the-pile TBW (durability) rating. It actually makes sense.

The Backstory

I have a 2017 iMac (bought that year as well) which I use as my primary home computer for photo/video editing, a bit of programming and a lot of Netflix and HBO Go as well. About a year ago, I got myself a bunch of disks and Bluetooth headphones in a short span of time.

I plugged in a 1 TB Seagate Backup Slim HDD for extra storage, along with a 2 TB Seagate Backup Slim for Time Machine backups. The two of them later got augmented with a 6 TB Seagate Backup Plus Hub HDD, which I partitioned into a 4 TB Time Machine volume and a 2 TB archival volume.

Roughly around the time that I made these “enhancements”, I started observing very frequent Bluetooth failures. My keyboard or, more often, trackpad would randomly disconnect.

The Suspects

Reading through countless posts on and, I came across the following top suspects:

  1. Too many devices on the Bluetooth interface
  2. Too much interference in the 2.4 GHz frequency range

I only had a Magic Keyboard 2 and Magic Trackpad connected wirelessly to the iMac, along with my Bluetooth headset. So, I got rid of the thing that was disposable – the headset.

No improvement.

I switched from the Magic Trackpad to Magic Trackpad 2 (mostly because the former used AA batteries while the latter has internal rechargeable battery). That… didn’t achieve much either.

Back to the Disks

This is a post about backup disks. Why are we digressing? On the disk side, I changed things a little bit – the 1 TB Backup Slim was replaced with a 1 TB Samsung T5 Thunderbolt SSD. Not a speed demon like the NVMe’s (that overheat quickly) but still 4x faster to write and 6x faster to read than the HDD.

I moved all my photo/video working data to the SSD and the system was significantly more responsive… except when it wasn’t due to the remaining two backup drives. The Backup Plus Hub frequently caused the iMac to freeze while spinning up.

I unchecked the “Put hard disks to sleep” option and that made a significant improvement. I was still mildly annoyed with the near constant rumble of disk activity but it was a good thing to have nonetheless. Ever since I started using Time Machine, I’ve had several minor accidents averted due to the backups.


Last week, I had another one of those issues when all my Bluetooth devices – keyboard, trackpad and mouse (wife’s preference) – simply refused to connect. I reconnected them one-by-one using a lightning cable but the trackpad kept failing. I tried resetting Bluetooth using the debug menu (⌘⌥⇧-click on the Bluetooth icon in status bar) and then it struck me – during all this drama, the Backup Plus Hub was furiously busy writing.

A bit more of “googling” soon landed me at a lengthy discussion where people were discussing how USB-A devices were causing Bluetooth connections to fail – some claiming interference, others claiming sleep/wake issues. I looked at my iMac’s 4 USB ports and they were all occupied – three without any devices on the other end but one with the Backup Plus Hub.

The big noisy USB-A HDD had to go.

The Replacement

If you’re thinking about replacing your backup HDD with an SSD, the first thought is the price shock. In terms of cost per TB, even a SATA III SSD could be 8x the price! Is the price really worth it? For a backup disk?

To answer that, we need to know what the expected benefits of an SSD for Time Machine would be.

Aside from the sluggish sleep/wake cycle that causes a mac to occasionally freeze, the other aspect is the frequent disk activity that always puzzled me for a drive that just took hourly backups. When I investigated this, it turned out that there’s a lot of Spotlight indexing activity that goes on in the Time Machine volume. And, as of Mac OS Catalina, it is impossible to disable indexing on a Time Machine volume.

So, the Time Machine disk will be busy most of the time.

Talking about price per TB, one of the new options, especially for high capacity drives is the QLC (Quad Layer Cell) SSDs that are about half the price per TB compared to “regular” SSDs. If you read up about it, you’ll find that these SSDs have a longevity compromise.

Data storage cells in SSDs degrade a bit every time they are overwritten. Multi-layer cells degrade faster. Thankfully, manufacturers now publish a metric called TBW (Tera Bytes Written) in the SSD specifications to indicate just how much data can be reliably written to the disk before its cells start to fail.

The 4TB Samsung 870 QVO has a TBW rating of 1440. Is 1440 TB of writes a high number or low number? Perhaps it’s good to decide on a life expectancy for a drive. I pick 10 years – that’s the amount of time in which any data written to the SSD would potentially fade out. This duration is different from degradation due to rewriting. It’s also a reasonable duration after which the existing disk size would be deemed insufficient for future use, making the disk’s replacement imminent.

1440 TBW over 10 years translate to 16GB of data written per hour per day every day for those 10 years. My Time Machine backups, on the other hand clock in at ~ 800MB per hour on average. That’s a whopping 20x headroom in longevity measured by TBW for a “cheap” SSD.


I’ve had my 870 QVO attached through a USB 3.1 Gen 2 interface along with the T5 for about 2 days now. Things are looking good. The trackpad even feels more responsive, where it used to be sluggish earlier on. It took a bit of “challenging the convention” to go with the 870 QVO but it’s delivering on the promise of improved responsiveness from the iMac and more stable Bluetooth peripherals.