Solved Macrium Reflect: Strange Differential Backup sizes

I'm using the Home Version 8.0.6867 and I always perform a full back every Sunday, with daily scheduled differentials for the remainder of the week.

Occasionally, one of the differential backup sizes increases to a disproportionate figure - see the attached image from Sunday through today - when there has been hardly any usage on my laptop. No Windows updates, no programs added or deleted etc.

I just wondered if anyone could shed some light on it, please?

This explains it (?) From macrium help:

ifferential Images​

A differential image stores the changes that have been made to the imaged file system since the last full image. Subsequent differentials can be taken, but only one differential and the full are required in order to fully restore the system.
This is quicker than creating a full image, however the longer the time between the full and the differential, the larger the differential image file is and the longer it takes to create.

Advantages of differential images:

• Differential images created after the initial full Image are very quick because only file system changes since the full backup are saved.
• The amount of disk space used by differential images is significantly less than that of full images.
• Only two image files are required to restore the system.

Disadvantages of differential images:

• As the time since the last full image was taken increases, the size of the differential grows as does the time it takes to create the differential image.
• In order to reduce this time, it is necessary to perform a full image occasionally to reduce the size of the subsequent differential images.

Hi Davehc. I have seen that 'explanation' on their website, but a 5GB difference in one day doesn't make sense, does it? Especially when I hardly used my machine at all yesterday...

Peter B said:

Occasionally, one of the differential backup sizes increases to a disproportionate figure - see the attached image from Sunday through today - when there has been hardly any usage on my laptop. No Windows updates, no programs added or deleted etc.

I just wondered if anyone could shed some light on it, please?

Click to expand...

Each differential has to be larger than the previous one, as it has to include all previous changes since the Full, plus any new ones.

As you have Home you could use Incrementals instead. These only include changes made since the last backup, be that a Full, Differential or Incremental.

In addition, if the partition being backed up has been defragmented since the last backup then the sectors holding the files have changed, even if the files themselves have not. All these changes have to be included in the next Differential/Incremental. On all my machines with HDDs I have turned off scheduled optimisation to avoid this.

This should normally only be an issue if the drive is an HDD, an SSD should not need to be defragmented.

Peter B said:

Hi Davehc. I have seen that 'explanation' on their website, but a 5GB difference in one day doesn't make sense, does it? Especially when I hardly used my machine at all yesterday...

Click to expand...

5 GB in a day can easily happen e.g. temporary files not yet cleaned up.

Thanks for all your replies, but I'm still convinced there's something not quite right...it only occurs once in a blue moon, however, so it's no big deal.
During a 'normal' week, it always increases accumulatively during the 7 day cycle, but usually by expected amounts - for example, the difference between the previous Sunday through Saturday was only around 2GB in total in 6 days...and during that period, there were Windows and Program updates...

Don't be too quick to dismiss what Bree noted. Have you actually checked to see if a Defrag happened between those days? As noted by Bree, this would tend to specifically affect a HDD rather than an SSD. Let me explain how this works and how to check...

So, let's assume that you have a bit of fragmentation and you perform a full backup today. Sometime after that backup a Defrag gets performed. The next day a differential is performed but you have not changed a single file nor have you added any files at all to the system, but you still have a large differential backup. That can be absolutely normal. Remember, Macrium Reflect disk image backups are BLOCK based backups, NOT file based backups. So, if you have a 4Gb file that wasn't altered by you, but it was defragged, any blocks altered by Defrag will have to get backed up. This could result in up to 4Gb of backup even though you never altered a single byte.

I'm not saying for sure that is what happened here, but let's check. In File Explorer, right-click the drive that you are backing up and select Properties. Go to the Tools tab and select Optimize. When was the last time that it was optimized?

If that turns out to be the cause, it's easy to work around. Start by disabling the automatic schedule. Then run a Defrag / optimization after the last differential for the week but before the full backup by using Task Scheduler to perform a Defrag using command line commands. You can run Defrag /? To see the syntax. Since the full backup will backup everything anyway, we don't care if a Defrag changes a lot of blocks before it runs.

Also, be aware that Macrium Reflect image backups are unaffected by changes to temp files such as the page file or hibernation files as the blocks occupied by these are excluded from backups.

So, after all that - if this turns out not to be the cause of the issue let us know and we'll see if we can determine any other cause.

Sorry, I didn't mean to sound dismissive - I hadn't considered the defrag/optimisation effect. My SSD was trimmed at 17.50 yesterday, so that's probably the answer, thank you all.
I'll look at an optimisation schedule, or only perform it prior to a backup.

Peter B said:

Sorry, I didn't mean to sound dismissive - I hadn't considered the defrag/optimisation effect. My SSD was trimmed at 17.50 yesterday, so that's probably the answer, thank you all.
I'll look at an optimisation schedule, or only perform it prior to a backup.

Click to expand...

If you run into any more issues, give a shout back. BTW, I'm sure no one thought you were being dismissive. Sometimes it's just hard to wade through all the available info