What carries over when you sign in to a borrowed Mac
· 5 min read
You are working on someone else’s Mac for an afternoon and want a file from iCloud. The obvious move is to sign in, get the file, and sign out.
That works, and it does considerably more than fetch a file. Knowing what is worth understanding before you do it on a machine you do not control.
What signing in actually does
Signing in to iCloud on a Mac turns on sync for whatever categories are enabled by default. Depending on the machine’s settings, that can include:
iCloud Drive, which begins downloading your files to that machine.
Photos, which begins downloading your library. On a large library this can be tens of gigabytes and it starts immediately.
Messages, which syncs your message history to the machine and starts delivering new messages to it.
Contacts, Calendar, Notes, Reminders, all synced locally.
Keychain, if enabled, which puts your saved passwords on the machine.
Find My, which adds that Mac to your device list.
None of that is hidden or malicious. It is what signing in means, and on your own machine it is exactly what you want. On someone else’s it is a copy of your personal data on hardware you will hand back.
What signing out removes, and what it does not
Signing out prompts you about keeping a local copy of iCloud data. Choose to remove it and macOS deletes most of it.
What is less reliable is everything that touched the machine along the way. Cached files, thumbnails, Spotlight index entries, and anything an application copied into its own storage. macOS does a reasonable job and it is not a guarantee, and the machine’s owner has an administrator account and can look.
Messages in particular tend to leave traces, and a signed-out account can still leave notification history.
The safer route
For getting at one file or checking something, do not sign in at all.
Use icloud.com in a private browser window. Full access to Drive, Photos, Mail, Notes and the rest. Nothing syncs to the machine, nothing is stored locally beyond the download you deliberately make, and closing the window ends it.
Sign out of the site and close the private window when finished. That is the whole cleanup.
For a single file, use a share link generated from your own device beforehand. Nothing to sign in to at all.
For a longer session, use a guest account on the borrowed Mac if the owner offers one, then icloud.com within it. Everything disappears at logout. Guest mode covers what that does.
If you do sign in
Sometimes you genuinely need the machine to act as yours for a few days. If so, reduce what comes with you.
Sign in, then immediately go to System Settings, your name, and turn off everything you do not need. Photos and Messages are the two worth disabling first, because they are the largest and the most personal.
Keep iCloud Drive on if that is why you are there, and turn off Optimise Mac Storage so you are not leaving downloads behind unpredictably.
Turn on Find My if you want the machine listed while you have it, and remember to remove it afterwards.
Signing out properly afterwards
- System Settings, your name, Sign Out.
- Choose not to keep a copy of iCloud data on the Mac.
- Sign out of Messages and FaceTime separately; they have their own sign-in.
- Sign out of Safari or whatever browser you used, and clear its history for the period.
- Check the Downloads folder and the Trash, then empty the Trash.
- On your own device, check Find My and remove the borrowed Mac from your device list.
Step six is the one people miss. A machine you no longer have sitting in your device list is untidy and it is also a device that can receive your two-factor prompts.
Two-factor codes are the real consideration
Worth stating separately, because it is the part with lasting consequences.
A Mac you sign in to may become a trusted device, which means it can display verification codes for your account. Removing it from your device list afterwards is what undoes that.
If you are signing in to a machine belonging to someone you do not know well, this is the strongest argument for using icloud.com in a private window instead.
Questions
Does using icloud.com leave anything behind? A private window leaves no history or cookies after closing. Files you download are still in Downloads, so remove those.
Can the owner see what I did in my account? Not while you are signed in, since your account is separate. After you sign out, remnants can remain on disk, which is the argument for not signing in.
What if I forget to sign out? Sign in at icloud.com from your own device, go to Find My, select that Mac, and remove it. That severs Find My. For a full sign-out you need access to the machine, so ask the owner.
Is it different on a family member’s Mac? The technical behaviour is identical. The judgement about whether it matters is yours.