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I decided to set up my usual test partition with Catalina, and came across some things I sort of knew but hadn't given much thought to.

First, DiskUtility reminded me that it is better to set up a new Volume that a new Partition. While Partitions require their own dedicated space, Volumes share whatever space is available on the drive. So Catalina only takes up about 12G on my drive instead of the 60-80 I would have set aside on a test Partition.

Second, this procedure added two Volumes, one for the OS and one for "Data". The Volume for the OS is not writable, or more to the point, won't let me erase anything, so I'm stuck for now with all the junk apps that come with the OS. I suppose this could be an issue when setting up some apps that write to the OS folders. I'll be looking for a workaround to get rid of the junk.

Third, I found I didn't need to install the apps that are already on the other volume. They can be run from where they are. I tried DP, Logic, DSP-Q, Numbers, MuseScore, BBEdit and a few more. In all cases they went through their first run setup (e.g. DP asked for email, SN, etc). I don't know if everything will run but this is a nice capability. I recall in previous OS versions being told by the Apps they had to be in the current Applications folder.

Fourth, I found that both Volumes share a backup folder if the TM backups are placed on the same external drive. That's a potential problem (e.g. it has backed up my Photos lib after it was converted to the new Photos version, so probably not usable any more with the old Photos version). So probably a good idea to use a separate volume for TM on the test Volume.

I don't know whether Apple planned it this way, but it is a lot more practical to have multiple OS versions on the internal drive than it once was.

I'm running Catalina on my MBP instead of setting up a new Volume on my iMac Pro.

Question: Can you access the Repair Partition with two OS on your System drive? If Command r on start up doesn't take you there, then the answer is No.

If yes, I'll install a Mojave volume on the MBP and test APFS Snapshots and a few other things.

No, the repair partition doesn't come up from either startup volume. I didn't expect it to. If you have two startup volumes, presumably one can be used to repair the other, and these are no more or less likely to become corrupt than the repair partition itself.

Having played around with Catalina for a while now, I'd have to say I don't see any point in going through all the authorization, updates, and installations needed to set up a fully operational audio Volume. Not much wrong with it, but I didn't see anything that works better, and there are a few things that are still a bit clunky --- mainly the bundled Apple apps --- at the current 10.15.1 update.

bayswater wrote:..If you have two startup volumes, presumably one can be used to repair the other, and these are no more or less likely to become corrupt than the repair partition itself...

There are a few things that can be done only in the Repair Partition. The one that is important is a complete restore of the System drive via APFS Snapshots.

I'll stick to 10.14.6 on the iMP and 10.15 on the MBP for now. Some developers need to get off the stick (IKM) and I gots to spend some money before Catalina becomes practical for me. I think I waited two years to move to OS X—likewise from 10.5 to 10.6.

mikehalloran wrote:I'll stick to 10.14.6 on the iMP and 10.15 on the MBP for now. Some developers need to get off the stick (IKM) and I gots to spend some money before Catalina becomes practical for me. I think I waited two years to move to OS X—likewise from 10.5 to 10.6.

I'll be keeping the 10.15 volume for anything that might come along that needs it, but for the most part, I'll be sticking with 10.14 day to day. Looking at the 10.14 applications folder from the running 10.15 OS, there are a lot of things that won't run (have the "do not enter" sign) I'd want on a working setup, and even most of those are 64 bit.

It will be interesting to see how the new Pro does, if it ever appears, when there so much stuff that hasn't been updated.

bayswater wrote:...
It will be interesting to see how the new Pro does, if it ever appears, when there so much stuff that hasn't been updated.

Yes but since it’s being built for a specific AV user base, I don’t expect it to be that big of an issue for them. The rest of us... that’s a different matter.

I expect that, when the new NEW Mac Pro ships, a large number of iMac Pros are going to hit the used market. Apple is expected to announce ship dates very soon. It will be interesting to see if it will require Catalina or if there will be a way to install Mojave for those of us who won’t/can’t consider 10.15 yet.

In a way, I do wish I’d waited for the MP but I got such a good price on my used iMP that I would have been crazy to pass it up—and it really is all the Mac I need for now and the near future.