Running Mojave on my iMac w/Catalina on an external SSD

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Running Mojave on my iMac w/Catalina on an external SSD

I have Mojave on my iMac [2017] and want to keep it there, for now - I have created an external SSD drive to run Catalina = I can boot into either one via the Startup selection. All is fine that way. But I do have a Q or two =

I have Mojave iMac DesktopDocuments synced to iCloud - does that mean I cannot do likewise with the Catalina OS? Does the Catalina OS sync things to a separate folder on iDrive?
When I am booted into Catalina all is fine but when the external drive is on and attached to my Mojave OS iMac it comes in as two partitions == SSD itself and SSD-Data as well, roughly equivalent. There are not 'two partitions' when I'm booted into Catalina. Where can I find out more about this two partitioning and Data subfile.>>???

iMac 27″ 5K, macOS 10.14

Posted on May 24, 2021 7:20 PM

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Answer 1

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PRP_53

Level 10

85,567 points

May 25, 2021 7:19 AM in response to Victoria Herring

The Volumes on APFS Formatted Drive are able to expand and / or contract as Space within the Volume is required. That is a feature Apple built into the new System Filing system ( APFS ) The Drive itself - is preset to - as you mention 1 TB and will never change.

If my personal illustration my assist your your purposes. Have a Mac Mini running High Sierra, an iMac running Catalina and a MacBook Air Newest M1 CPU from Apple running Big Sur. All three devices are using the same Apple ID and All are using iCloud. All the changes created on the High Sierra appear on the other two devices running newer version os macOS. Any changes on the Big Sur are reflected on the Other devices running older versions of macOS.

iCloud is just the conduit / means to sync the changes from one device to another as long as ALL use the Same Apple ID.

Hope this helps for your needs.

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Answer 2

PRP_53

Level 10

85,567 points

May 25, 2021 2:28 AM in response to Victoria Herring

Question # 1 - No the sync from Catalina and Mojave use the same folders as long as both are using the same Apple ID.

Question # 2 - Starting in Catalina and above - when Catalina is installed it creates Two Volumes that reside on the same Drive. This is achieved by using the the New APFS Filing System. Normally - the two Volumes are called Macintosh HD and Macintosh HD-Data or in your case SSD and SSD-Data. The reason Apple has done this is to separate the User Data from the macOS OS. In so doing making it more difficult for UnAuthorized changes to the OS.

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Answer 3

May 25, 2021 7:07 AM in response to Victoria Herring

That is very helpful and I appreciate your explanation of the volumes displayed. Why does the HD volume have so much space then? OS & Apps don't take that much room or is it just so each of the volumes has the same # but the total is always 1T regardless?

Appreciate the explanation ref the use of the folders == that should work out just fine - I assume that if for some strange reason the same named folder/item will just become #2 if it is saved also on the Catalina drive in the cloud?

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Answer 4

PRP_53

Level 10

85,567 points

May 25, 2021 7:34 AM in response to Victoria Herring

Welcome and Many Thanks to you too :-))

Ah - Mac Mini cira 2011, iMac late 2012. Mac mini connected to HD TV hdmi for Netflix and iTunes mostly. iMac for general stuff liker ASC posting and MBA for fun stuff and keeping me aware of the M1 CPU and Big Sur.

Very Best and stay safe COVID-19 and all.

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Answer 5

May 25, 2021 7:27 AM in response to PRP_53

Thank you -= the illustration is helpful [I'm a bit jealous but desk space wouldn't allow yet another Mini here, or two!] --

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Answer 6

May 25, 2021 7:46 AM in response to PRP_53

You too - and when/if I want to update [my iMac is 2017/Mojave] I'll keep your example in mind!

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Running Mojave on my iMac w/Catalina on an external SSD

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