Two Different OS on iMac

[alanmerbs]

I have a late 2013 27" iMac. The internal hard drive started acting funky so I added an external SSD. Now the internal is working OK for the time being so I'm wondering if I can put Catalina on the external while Mojave is still on the internal so that internal and external drives have different OS? Thanks!

 

[ruslan120]

Absolutely.

Just select the appropriate disk during installation and then hold the Option key as you turn the iMac on to select the disk to boot.

 

[alanmerbs]

Cool ...thanks!

 

[charlituna]

Absolutely.

Just select the appropriate disk during installation and then hold the Option key as you turn the iMac on to select the disk to boot.

what about the other way around. i find myself in need of being able to run el cap on my iMac for one app that doesn’t work in newer OS versions. But I need to keep Mojave on my computer for the other 6 programs I need to run.
can I put el cap on say a usb thumb drive and option boot to it when I need to run said app (I only need it like twice a week). will I be able to see my internal drive for the files, save to the internal etc. do I need to do anything special like make sure they are both formatted the same way (ie APFS or both journaled)

 

[ruslan120]

what about the other way around. i find myself in need of being able to run el cap on my iMac for one app that doesn’t work in newer OS versions. But I need to keep Mojave on my computer for the other 6 programs I need to run.
can I put el cap on say a usb thumb drive and option boot to it when I need to run said app (I only need it like twice a week). will I be able to see my internal drive for the files, save to the internal etc. do I need to do anything special like make sure they are both formatted the same way (ie APFS or both journaled)

Nope!