It used to be that when moving to a new Mac, your internal storage grew in capacity. I moved from a 1 TB internal hard disk to a 2 TB Fusion Drive. Unless you’ve got plenty of money to spare, upgrading your Mac is now likely to bring a reduction in that storage: my desktop systen has just dropped from 2 TB to 1 TB.

This is most likely to occur when you are switching from a Mac with an internal hard drive or composite (Fusion Drive), to one with an SSD. Prices of SSDs have been falling encouragingly, but they’re still significantly more expensive than hard drives, and Apple’s pricing policy makes internal SSDs considerably more expensive than externals.

For example, configuring a new Mac mini with a 1 TB internal SSD instead of the standard 256 GB increases its price by a huge £/$/€ 540 (on a base model costing as little as £/$/€ 799). For the same step in a third-party NVMe SSD, the price increment is currently just over £/$/€ 100, with a good 1 TB NVMe SSD costing between £/$/€ 150-200 in total. Apple makes a very handsome profit on sales of internal SSDs, many of which are now of course soldered in, so cannot be upgraded by the user.

All SSDs are not, of course, equal. In Macs with a T2 chip, that functions as the controller for the internal SSD, with in-line encryption, which performs in NVMe class, far better than you will get in any external Thunderbolt 3 case. Selecting the right size internal SSD and deciding how to spread your files between it and external storage are therefore critical.

When downsizing internal storage, the first thing that you need is a clear idea of what you have at present, so you can draw up a plan which keeps cost to a minimum, and performance close to optimum.

The best tool for this is DaisyDisk, although there are others which might be as good. Get it to scan your current internal storage, and see what is taking the space. Don’t whatever you do try using the Storage tab in About This Mac: it is more often wrong than right, and will give you a very misleading view of your data. It also works not by folder but by what it considers to be document type, which can be quite different.

If I had to downsize from this 1 TB SSD to 512 GB, the only way that I am going to be able to do so (without causing major problems) is by offloading the contents of the Users folder onto external storage.

Hone in then on the only user, and you’ll see that slightly more than half of the folder size is accounted for by my iTunes library.

Your downsizing plan therefore needs to identify sufficient space on your current storage which can safely be moved to external storage.

Folders within the Users folder are your best candidates. Although you can of course run apps from external storage, there are several good reasons for trying to avoid doing so. Apps which are bundled with macOS are now protected by SIP. Apps which are delivered through the App Store are best kept in their normal installation folder, which is Applications. Other apps can have problems connecting with support files, etc., if they are moved to external storage. And above all, your external storage is significantly slower to access, so you really don’t want to impose that overhead on applications. I have in the past, though, used external storage for little-used apps, when I had no choice.

So the contents of /System, /Library, /Applications, and the smaller hidden folders are going to remain on your internal storage.

If your ~/Documents folder is a good candidate for moving to external storage, then it’s likely to be your first choice. The great majority of the files within that change relatively little. Performance-critical files such as databases – and in my case, my Xcode projects – can be kept in the near-empty Home folder on internal storage. When carefully planned and executed, moving Home folder content to external storage can be a perfect solution.

I let initial setup configure my Home folder normally, and before migration set up my new internal Home folder on my old Mac. This required removing almost everything in ~/Documents, and all my photo libraries in ~/Pictures. Those are now in folders named Documents and Pictures on my external storage (a 1 TB SSD in a Thunderbolt 3 case for a fraction of Apple’s cost).

My main working folder within that external Documents folder is then linked by a Finder alias into my internal ~/Documents folder. When I select the Documents item in Finder’s Favourites, this contains the alias which takes me straight to my external working folder. Anything which can follow the path linked by that alias will then see the path to embedded images, etc., unchanged.

Setting up Photos, iTunes and other libraries on external storage is usually straightforward provided that you remember to set the new location in Photos’ or iTunes’ preferences. If you are uncertain how to handle shared libraries or other details, Apple describes the process in detail here for Photos libraries.

Moving your iTunes library is even simpler: just copy across the folder ~/Music/iTunes together with any other folders in which your music and media are stored. Once that’s complete, remove the originals. Then start iTunes with the Option key held down, so that you are prompted to select your new iTunes Library location.

Your goal at the end of all this is to have an internal SSD which is around half full, if possible, giving it room to grow over the coming few years. External storage can be used more fully, of course, as it is easily replaced with larger capacity once you have recovered from the financial shock of replacing your Mac.

In the final article, I will try to assemble some useful charts to guide you through moving to a new Mac.