External storage is invariably sold with ‘up-to’ performance figures. In practice, you’ll seldom realise anything like some write or read speeds claimed. And when it comes to prolonged tasks like that first full Time Machine backup, no matter how fast you thought that drive would be, it always takes longer than expected.

Over the last few years I have tested and reviewed many examples of different types of external storage, from basic USB 3 hard drives, to the latest USB4 SSD enclosures, and NAS packed with fast SSDs. This article draws on all those test results to give you a better idea of what to expect when they’re being used with your Mac.

Results quoted here are typical for those tests performed mostly using a Mac Studio M1 Max, but unless otherwise indicated should be similar for recent Intel models. They’re summarised in this table.

Write speeds are given for:

• the single 50 MB write test performed by Time Machine before each backup;
• 500 multiple concurrent writes of 4 KB each, performed in those same Time Machine tests;
• calculated net write speed over a first full backup to APFS of at least 400 GB;
• general write speed measurement using my app Stibium, which gives broadly similar results to other leading benchmarking apps.

General read speeds are also obtained using Stibium, and similar to other apps. All speeds are given as MB/s for consistency.

Before looking at individual types of storage, one obvious and important result is the effect of throttling by macOS on Time Machine backup performance. Considering Time Machine’s own tests, writing a single 50 MB file is performed consistently at around 200-225 MB/s to local storage of whatever type, and multiple concurrent writes of 4 KB files reach around 20-23 MB/s regardless of local storage type. Those hold good even when you back up to a fast Thunderbolt 3 SSD, and backing up to a NAS is little quicker unless it’s over 2.5GbE to an NVMe SSD. Local transfer speeds only differ more substantially in general tests, when they aren’t throttled as they are in Time Machine.

Hard disks

When writing to or reading from a local hard disk, performance varies substantially according to which sectors on the hard disk are being accessed. This is a well-known phenomenon, and the result of geometry, as sectors are faster at the periphery of the disk’s platter, and slower in the inner part. Ranges given here take that into account: the lower figure is for inner sectors, and the higher for outer ones. Some users compensate for this effect, and only ever use the outer half of a disk’s sectors to obtain better performance, but that reduces their available capacity, and effectively doubles their cost per TB.

SSDs

SATA SSDs may be cheapest, but they’re also slowest, and with Macs they generally don’t enjoy Trim or SMART health indicator support. Of the two, Trim support is usually the more important, as without that, they can accumulate blocks waiting to be erased and returned for further use, and as a result their write (but not read) speed can fall as low as 100 MB/s. Unless used for largely static storage, this is a significant risk.

NVMe SSDs deliver twice the performance of SATA models, and generally enjoy Trim but not SMART indicator support. This makes them far better suited to general use, as their write speeds should be sustained from new throughout their working life.

USB 3.2 Gen 2, Thunderbolt 3, USB4

Translating commonly quoted transfer speeds for these three protocols into real-world speeds turns out to be complex. In practice, these are what you can expect to see:

• USB 3.2 Gen 2 at 10 Gb/s is slightly less than 1 GB/s
• Thunderbolt 3 at 32 Gb/s is up to 3 GB/s
• USB4 at 40 Gb/s is up to 3.4 GB/s.

All recent models of Mac, both Intel and Apple silicon, should realise full performance over USB 3.2 Gen 2 and Thunderbolt 3, but support for USB4 is limited to Apple silicon. Unless a drive or enclosure specifically includes Thunderbolt 3 as a fallback, when connected to an Intel Mac, you should expect it to fall back to USB 3.2 Gen 2 at just under 1 GB/s, less than a third of the speed of USB4.

NAS

Although I haven’t made any systematic comparison between AFP and SMB network protocols, I can see no consistent difference in their performance, when used with the latest versions of macOS and NAS software. The latter, though, can be critical: older versions of NAS software can perform poorly when used over SMB with recent macOS. Keeping your NAS software up to date is important.

Throttling of Time Machine backup writing isn’t supposed to occur when backing up over a network, and there is some evidence here to support that, with significantly better results for 50 MB test files. However, those are only apparent when using NVMe SSDs in the NAS, with a wired Ethernet 2.5GbE connection to provide sufficient bandwidth.

Check TM performance

Provided that your Mac is running a recent version of macOS and backing up to APFS, it’s simple to read the two write performance tests that occur at the start of each Time Machine backup using my free T2M2. Alternatively, you can also read them using the Time Machine custom log extract in Mints. In T2M2 they should look something like:
Destination IO performance measured:
Wrote 1 50 MB file at 238.02 MB/s to "/Volumes/ThunderBay2" in 0.210 seconds
Concurrently wrote 500 4 KB files at 35.58 MB/s to "/Volumes/ThunderBay2" in 0.058 seconds

Check general performance

Although there are other apps that will do this, I developed Stibium for this purpose. Follow the ‘gold standard’ procedure detailed in its Help Reference to obtain the most accurate and reproducible results. Stibium can test any storage you can access in the Finder, including all local devices and networked systems such as NAS.

Further reading

Which external drives have Trim and SMART support?
How to evaluate an external SSD
You can read my reviews in MacFormat and MacLife magazines, available in the App Store.

The most difficult question when setting up a backup scheme is determining how much storage you need for all those backups. This article suggests how you can estimate how big that backup storage needs to be, assuming that you’re using incremental backup, like Time Machine, which keeps a sequence of older versions of each file in a series of multiple backups.

Basic formula

At its simplest, you need to estimate the size of the initial backup, and the average size of new backups each month for the period you intend using that storage. Then work out
initial backup size + (average monthly backups × number of months taking new backups)

Let’s say my Mac has a 1 TB internal SSD, with around 500 GB to be backed up, and each month 50 GB will be added to that. If I want to use that as my backup storage for 2 years, then it will need a capacity of
500 + (24 x 50) = 1700 GB
so a 2 TB drive should do fine for slightly longer.

In practice, Time Machine and other backup utilities are more frugal with the space they require, as they automatically ‘thin’ or remove some older backups. Time Machine’s policy is to keep:

• hourly backups for the previous 24 hours, in addition to hourly local snapshots on each volume being backed up,
• daily backups over the last month,
• weekly backups back to the start of that backup series.

Initial backup size

When backing up data-only volumes, like media libraries, almost everything on them is likely to be backed up initially. Significant exceptions include hidden folders containing Spotlight indexes and version databases; the former can be substantial, and can be measured by showing hidden files in the Finder, selecting the .Spotlight-V100 folder, and inspecting its size in the Get Info dialog. The latter can only be measured in Terminal, and can be safely ignored. Another hidden occupier of free space are snapshots, whose size can be estimated in Disk Utility.

Estimating backup size from a boot volume group, such as that containing your Home folder, is far harder. Time Machine and other backup utilities will only back up the Data volume in any case, and it usually holds many files and folders that aren’t backed up, in addition to its Spotlight indexes and version database. You may be able to gain an estimate from the Storage item in General settings, although that tends to lump a great deal into its System Data, some of which will be backed up, and some won’t.

If you use iCloud Drive much, particularly if you keep your Desktop and Documents folders in iCloud Drive and Optimise Mac Storage is enabled so that many files are evicted from local storage, you’ll need to consider how that might change if many of those items are downloaded, and can then be backed up. Another consideration is whether your backups will preserve sparse files at their reduced size, or expand them to full size. Again, that’s difficult to estimate, although Time Machine will normally retain their small size when backing them up to APFS.

Incremental backup rate

If you’re already backing up a volume, and it’s likely to see similar use in the future, use records of recent backups to estimate their daily or weekly total size. For Time Machine, you can do that using my free utility T2M2 (The Time Machine Mechanic). This provides:

• current free space in backup storage, which you can track over a week or so;
• actual free space and thinning size each time an old backup is thinned;
• total file size copied in each backup.

Save those records each day for a week, and you’ll get a very good idea as to how much additional space is required for the period you’re intending to keep those backups in that store.

Full backup storage

Another decision to make is what you’re going to do when that storage is full. If you’re using Time Machine, those backups are stored as snapshots, so at present can’t be copied across to another disk to retain in your archives. The only options are to reformat the volume for reuse, or to retain it as an archive. If you’re going to reuse that disk, consider whether it will still be large enough for use when it comes to the end of its backup series.

Summary

• Estimate initial backup size from the total used by all volumes to be backed up.
• Your boot volume group won’t require as much backup space as it uses, and Storage settings may help you guess how much to reduce that.
• Estimate the rate of growth using current backups, where available, from reports in T2M2.
• Decide how long you want to back up to that storage, thus how much your backups will grow over time.
• Then Total space required = initial backup size + (monthly increment × number of months in use).