Before you upgrade: the checklist that prevents most failures

Most upgrade failures are not caused by the operating system. They are caused by something that was not ready: no working backup, not enough free space, an app that silently stops working, or a machine that cannot be restored.

This is the checklist worth doing before you click install.

Before anything: prove the backup

A backup that has never been restored is a hope. Restore a single file from it, open that file, and confirm it is the version you expect. If the backup is to a network drive, confirm the drive is reachable and the last run was recent.

Check free space

A major upgrade needs room to download, unpack, and keep the old system for a while. As a rule, leave more free space than the download size times three. If the disk is nearly full, clear it before, not during: a failed install with a full disk is the worst version of this process.

Inventory what you actually use

Open your applications folder and write down the five things you would refuse to live without. Then check each vendor's supported-version page. This takes fifteen minutes and catches the majority of real breakage, because the apps that break are the ones nobody updated.

Know your rollback

Find out whether you can install your current version again, and whether your model still has a bootable installer path. Keep one if you can. The window closes when Apple stops signing the older release, and it does not reopen.

Write down what you are on

Record the exact version number before upgrading. Restoring to "the previous one" is not a plan; restoring to a specific build is.

Then install, and check four things

After the upgrade: does the machine boot without a login loop, does the network work, does your backup still run, and does the one app you depend on for money still open. Those four answers decide whether you continue or roll back, and it is much easier to check them immediately than after a week of work.

What not to do

Do not upgrade the night before something important. Do not upgrade while travelling. Do not upgrade a machine that is your only device without a tested backup. None of these are technical constraints; they are the difference between a boring evening and a lost week.

After the first week

Expect the small things: a driver that needs reinstalling, a shortcut that moved, a preference that reset. None of them are failures, and all of them are easier to handle when you know they are normal.