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I'd like to use a clean install of the latest Big Sur (11.4 or 11.5) on a machine that currently has Yosemite installed. I'd like to keep using Yosemite due to some legacy apps/licenses.

  • Ideally I'd like to edit the same files from both Yosemite and Big Sur
  • I've also considered accessing data and installing/running an app on a different partition to the one with the OS
  • However, I'm realising that there could be challenges due to the HFS and APFS formatting

I have Yosemite (10.10.3) installed on the Macintosh HD partition. I have two other partitions which I use as data storage (e.g. Word, Excel files).

P1) Macintosh HD (480GB, Yosemite already installed)
P2) 2 - 430GB
P3) 5 - 90gb

Scenario (i):

P1) Macintosh HD (480GB, Yosemite already installed)
P2) 2 - 430GB (Big Sur would be installed)
P3) 5 - 90gb

Scenario (ii):

P1) Macintosh HD (480GB, Yosemite already installed)
P2) 2 - 430GB 
P3) 5 - 90gb (Big Sur would be installed)

All three partitions according the disk util are:

   File System Personality:  Journaled HFS+
   Type (Bundle):            hfs

Question that arises:

  1. With reference to the two scenarios above, is it possible to access (both read & write) files (e.g. a Word document) and or run apps stored on a different partition than the one with the running OS?...Or will the partition (with the other OS installed) no longer be visible?

A.) If yes, how does this work if one of the drives is JHFS+ and the other is APFS?

B.) If not, would it be better to store data on a partition that doesn't have any OS installed (used almost like an external SDD except that it's internal)?

C.) Would there be any issues from installing apps not on the same partition as the one with OS-installed? E.g. app files that need to be located in the system, caches, application support and library folders

Machine: MacBook Pro (Retina, 15-inch, Mid 2015) 1TB SSD (3 Partitions):

/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            479.5 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
   4:                  Apple_HFS 2 - 430GB               430.3 GB   disk0s4
   5:                  Apple_HFS 5 - 90gb                89.7 GB    disk0s5

I've made an attempt to answer my question below but I'm still unclear on a few points (I've added my research with links there).

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    In any case, I'd recommend backing up anything you don't want to lose (and maybe making a USB installer disk for Yosemite, in case you need to reinstall it). I've been seeing too many "I repartitioned/installed/whatever and now nothing will mount/boot" type problems lately. Jul 16, 2021 at 1:16
  • @GordonDavisson Very valid point about backing things up. I have a time-machine backup. As the machine came with Yosemite, wouldn't there always be the option of booting from Recovery HD (which is/stores Yosemite)? Jul 16, 2021 at 7:40
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    If the local partition table gets messed up (a common problem), the local Recovery HD will be inaccessible (maybe even overwritten). You can use Internet Recovery with Command-Option-Shift-R to get the version that came with the Mac, or something close to it... maybe. I've heard of problems with that too. If you have a TM backup to a USB (or FireWire) drive you can boot from that and either do a full restore (if you back up everything including the OS), or run the install from that and I think it'll install the same version (but I haven't tested that theory). An install disk just works. Jul 16, 2021 at 8:02
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    There are links here to download old installers (actually, installer packages which will create the installer application, which is what you need to create an installer disk). I'm not sure which version of Yosemite it'll install, but I'd guess the latest. BTW, David Anderson posted several alternate methods for creating installer disks under this question; check them out if you run into any trouble. Jul 16, 2021 at 10:23
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    As posted by @GordonDavisson, use this link to download Yosemite and create the Install OS X Yosemite application. This should be the latest version of Yosemite, which should be compatible with your Mac. You will need to use your existing Yosemite to create the Install OS X Yosemite application. You can use this link to create the bootable USB Yosemite installer. The command for Yosemite has been omitted. You will have to adapt the command for El Capitan. Jul 16, 2021 at 15:19

1 Answer 1

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Here's my attempt to answer but still not very clear on a few things.

For maximum usability of the dual-boot with minimum performance/bug risk, I'm considering going with scenario (ii) with partition 3 formatted as APFS.

This way, I'd be able to store more data (430GB instead of 90GB) that is visible (and accessible, ie. read & write) when running Yosemite and also when running Big Sur:

  • Yosemite can see P2 but not P3
  • Big Sur can see both P1 and P2

In this case, I'd assume 'seeing' is good enough for read & write, if I'm not mistaken.

Another option would've been to have all partitions on APFS but apparently Yosemite doesn't work on APFS? Alternatively put everything in HFS+ but risk not being able to update Big Sur?

Links I've had a look at:

  1. Dual boot dual partition and clean install of High Sierra [@klanomath & @Chains here discuss using CLI to install Big Sur in HFS+ format]
  2. 10.13 to Big Sur - HDD APFS System volume and HFS+ User Space [@bmike seems to discourage Big Sur on HFS+...suggesting potential bugs and or performance/setup issues, if I understood correctly]
  3. https://www.reddit.com/r/MacOS/comments/odc717/dual_partition_boot_disk_with_yosemite_and_big_sur/ [I think what's explained here is even if APFS and HFS+ partitions are on the same SSD each partition won't be visible to each other]
  4. https://support.apple.com/en-us/HT208891 [having two APFS volumes on the same partition?]
  5. https://www.macworld.com/article/234913/macos-big-sur-supports-time-machine-on-apfs-formatted-drives-but-there-are-a-few-catches.html [Time Machine can't use APFS smoothly?]
  6. https://mrmacintosh.com/macos-big-sur-full-installer-database-download-directly-from-apple/
  7. https://www.youtube.com/watch?v=zHfJ0uHgktA [2 Easy Ways to Convert HFS/HFS+ to APFS Without Losing Data] - I don't think this is applicable to Yosemite.
  8. https://discussions.apple.com/thread/8570503 ["If the Mojave partition is formatted as APFS, Yosemite won't see it." and "Avoid placing Mojave on hfs although possible: any minimum upgrade to Mojave requires it to be on an Apfs volume." — I assume these apply to Big Sur too]
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    I agree. Do your scenario 3. Try and ensure that your user has the same ID on both Yosemite and BS to simplify permissions on the shared volumes. The first user created has ID 501.
    – Gilby
    Jul 16, 2021 at 11:03
  • Gilby thank you for your input!! I've only created one user and it indeed it has the User ID of 501 — I used the whoami and id commands in Terminal to confirm this. @bmike has mentioned here apple.stackexchange.com/questions/356716/… that home folder & apps are very portable (I'm assuming without the need for symlincing here) but that there might be "less functionality and speed" when moving and running apps from another location...have you encountered anything like this? Jul 16, 2021 at 12:34
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    Avoid symlinking home folder with Big Sur. BS does more (undocumented) things with ~/Documents, ~/Desktop which makes symlinking cause problems. Use Sys Prefs - Users and Groups to move the whole home folder. But, if it were me (because the Yosemite and BS are so different) I would leave home folders on the two boot drives and create folders to be shared on the third drives. Either way there will be challenges with OS versions so far apart.
    – Gilby
    Jul 16, 2021 at 22:49

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