Q I have installed a 64 GB SSD drive from which to boot my iMac. How do I now make Photos, iTunes, and Mail save their files to a large external disk, to stop the SSD from becoming overloaded?
A There is no common solution to diverting the main storage areas for applications to a volume other than the startup disk.
The simplest to deal with is iTunes: open its Preferences, click on the Advanced tool, and the top section of the dialog allows you to select a different location for its Media folder.
Photos will open any of its libraries simply by double-clicking on the library. So your best plan is to move the library that you wish to use onto your external drive, then double-click it. Once Photos has opened the library, set it as the System Photo Library in its Preferences dialog, and the next time that you start Photos, it should open that by default. You can confirm that in the current library location in its Preferences dialog.
Mail not only has no control itself, but there does not appear to be a third party tool to assist. You might be able to convince it to store your mailboxes through an alias to a folder on your hard disk, instead of its Mail folder inside the Home folder’s Library. But that could fail or break easily, and it is best if mailboxes do not grow so large as to make this a worthwhile gamble.
Updated from the original, which was first published in MacUser volume 27 issue 14, 2011.