Problem booting into Older Yosemite from Mojave on Mac Pro

[quadra605]

I work in sound editing & only update my OS when I have to. I have an older Mac Pro I use as the testing ground for new OS's BEFORE I upgrade. It's an Early 2009 MP 4,1 (firmware upgraded to 5,1) 2.66 GHz Quad-Core, 32GB RAM, Radeon RX560 4GB GPU, & the system drive is a Samsung EVO SSD mounted on an a PCI Accelsior card.

I also have another SSD mounted in one of the the hard drive bays partitioned to hold two alternate OS's. Most recently, the PCI-mounted system drive has been running Mojave & the alt drive has High Sierra. This weekend, I needed to access some old iMovie6 HD sessions and since that app doesn't open reliably in either Mojave or HS, I needed to use something older. No matter what I tried, I could not get an HD with OS X Yosemite to mount unless I installed my old nvidia 512mb Apple GPU. And when it DOES mount, it won't show the Mojave HD on the desktop (or in Startup Disks in system prefs).

I'm used to seamlessly booting back & forth between old OS's, but with the combo of Mojave, the need for a metal-capable GPU & this older OS that won't install on an APFS formatted drive, I think I'm overlooking something amongst all these variables. Anyone have any ideas?

ps--I'm aware that iMovie 6 HD requires a hack or hacked version 6.5.1 in Yosemite in order to launch.

 

[saudor]

The RX 560 requires at least sierra 10.12.6 to work properly
The nvidia wont work in Mojave or later because it needs metal (Unless Mojave is patched but doing this will break rx 560 compatibility)

Mojave is likely installed on a APFS partition which requires at least 10.12.6 sierra to read but preferably 10.13.

Might be a better idea to install an older version through a virtual machine(although there would still be no graphic acceleration and the OS will run very poorly) A Kepler based nvidia card can run both Yosemite and Mojave though. Just avoid the 650-660ti variants as they have issues in HS or higher

 

[quadra605]

The RX 560 requires at least sierra 10.12.6 to work properly
The nvidia wont work in Mojave or later because it needs metal (Unless Mojave is patched but doing this will break rx 560 compatibility)

Mojave is likely installed on a APFS partition which requires at least 10.12.6 sierra to read but preferably 10.13.

Might be a better idea to install an older version through a virtual machine(although there would still be no graphic acceleration and the OS will run very poorly) A Kepler based nvidia card can run both Yosemite and Mojave though. Just avoid the 650-660ti variants as they have issues in HS or higher

Thank you for the suggestions. Would an RX 580 solve the problem?

 

[saudor]

Thank you for the suggestions. Would an RX 580 solve the problem?

no. RX580 and rx560 are from the same family and have the exact same requirements. If you want to use both OSes, your only option is a kepler card like the 670,680, etc. Even then, you'd still have the usual boot screen issues and would need use opencore bootloader if that is a problem. Also, it is likely that kepler support will be dropped after big sur

 

[Mac Hammer Fan]

Yosemite doesnt recognize apfs mojave disks, i suppose. If i were you i would install at least high sierra as saudor said