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Before you begin, connect your old macOS drive or a Time Machine backup drive to your computer.
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If you forgot to make a backup of your old SSD before upgrading, you can install it to an external SSD enclosure to transfer any data you need as shown in this guide, and then keep it as an external storage drive.
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Power on your Mac and open the Migration Assistant application.
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After the Migration Assistant window opens, do not press Continue until you are ready to begin the migration process. Pushing continue will automatically force close any open applications and log you out.
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After you are logged out and taken to the Migration Assistant screen, select the first option: transfer From a Mac, Time Machine backup, or startup disk.
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Click Continue.
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Migration Assistant will list any connected macOS or Time Machine drives across the top of the window.
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Select the drive you want to restore from, and click Continue.
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If the drive you have selected is a Time Machine drive, Migration Assistant will list all the backups available from which you can transfer information.
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Select the backup you'd like to transfer information from, and click Continue.
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Select all the items you'd like to transfer to your new drive.
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Next, Migration Assistant may prompt you to enter a new password for the account on the new drive and/or the old password for the account on the old drive.
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Once you begin the transfer, a loading bar will appear and cycle through each of the data categories that you selected on the previous screen (applications, documents, etc.).
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Your computer may need to restart multiple times during the transfer process.
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When the transfer is complete, the Migration Complete window will appear.