A Mac can read an external drive but cannot save to it

Distinguish a Mac setting, an account issue and a hardware symptom

Record the macOS version, exact prompt or symptom and the affected app. Similar-looking problems can come from different places: an app’s permission, a system preference, a cloud account or physical hardware. Avoid changing several of these at once, because the result becomes harder to interpret.

Use a harmless test that preserves the original data. For file or capture problems, choose a known destination and check the resulting item in Finder. For account or credential prompts, identify the named account or keychain before resetting anything. Work-managed Macs may have profiles or required settings that should be reviewed with the administrator rather than bypassed.

After a change, repeat the task in the same app and account, then check a second ordinary task for unintended effects. Keep backups available before major repairs, storage changes or resets. Never include passwords, recovery keys or full credential dialogs in a public support request. If the symptom involves charging, heat, liquid or a physically damaged display, pause software troubleshooting and establish whether continued use is appropriate.