How do I save historic iMessages to external HDD?

Hello DocklandsBulldog,

Thanks for using Apple Support Communities!

We understand your iCloud storage is almost full and you want to move the iMessages stored in there to an external hard drive. We would be happy to provide some guidance on this.

At the moment, there is not a supported feature that would allow you to archive your messages from iCloud to a hard drive. If you wish to keep those messages for longer and continue to have available iCloud storage, you can look in to purchasing additional storage:

Buy more iCloud storage

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Best Regards.

As per this article https://support.apple.com/en-us/HT208532

If you turn off Messages in iCloud on an iPhone or iPad, your message history will be included in a separate iCloud backup.

Your iPhone, iPad, and iPod touch backups only include information and settings stored on your device. They do not include information already stored in iCloud such as  Messages in iCloud,

Also in your question - you said I'd like to archive everything prior to August 2017.

As per this article https://support.apple.com/lt-lt/guide/messages/icht20001/mac

Keep messages

Choose how long to keep message conversations in your account. For example, if you choose 30 Days, your conversations (and all attachments) are available in Messages for 30 days. After that, they are automatically removed.

Three years have gone by , if you click on drop down arrows for messages there is an option to keep forever .

But for the Mac , messages can't be literally viewed on iCloud.com they are synced with iCloud , and regarding moving them to external hard drive will not be possible , you can further send a feed back the link is already provided in the thread .

Also for archiving other data see the article https://support.apple.com/en-in/HT204055

See some articles to reduce the data https://support.apple.com/en-in/HT204247

https://support.apple.com/en-in/guide/macbook-air/apdb468896e0/2019/mac/10.14.5

https://support.apple.com/en-in/HT206996

https://support.apple.com/en-in/ghttps://support.apple.com/en-us/HT201318

https://support.apple.com/guide/system-information/find-and-delete-files-syspf5a64aa6/mac

Another option is buy a new storage plan https://support.apple.com/en-us/HT201318