I searched for this, and couldn't really find a related thread, just a bunch of people having a lot of issues with Mojave that really scare me! I've got an early 2015 MacBook Air running Yosemite. I have not "upgraded" until now because of audio hardware and software compatibility issues. I'm finally ready to give Mojave a try, since it's not totally ugly and I'm tired of Yosemite's networking issues. But I have some big concerns. I have a 15 year old G5 server that is running great, and I need to be able to share files and screens back and forth. It has my Adobe Creative Suite, so when I need to any graphic work (I'm not doing it professionally any more, so CS4 does everything I need and I don't want/can't afford to upgrade.) Anyway, I need my machines to talk, share files and screens. I also have to keep the MBA running Yosemite on the internal SSD functional while I get Mojave working, if it's even possible. I have an external GUID HFS+ drive with several partitions, and I'd like to install Mojave on one and get it all set up before I clone it over to my MacBook Air. I want my MBA's internal SSD to remain HFS+ so I can still access files from the G5 when I need to. So, here are my questions: 1) Will Mojave install on an HFS+ partition, or is it going to try to reformat the whole drive? 2) Will my MBA still run Yosemite after I do the firmware updates for Mojave? 3) Is there a way to upgrade the firmware without running the Mojave installer? I don't trust that the installer will leave my MBA's file system alone if I start it. 3) Is there any way install Mojave on an external GUID, HFS+ magnetic drive (not SSD)? I'd prefer not to install Mojave over an existing OS if possible, but I've heard that the only way to get it to install on HFS+ is if it is upgrading an existing OS. Is this true? If so, can I just install Yosemite on a partition on my external drive, upgrade that, get everything set up and working, then clone it over to my MBA's internal SSD when it's functional? 4) Will Mojave still be able to do screen and file sharing with my G5 (Leopard)? 5) If I do install Mojave on the external drive and clone it over, is it going to create a Recovery Partition that I can clone over as well? Or, is it going to need to reformat my entire external drive? In Yosemite, it was possible to get Disk Utility.app to see all partitions, but I've heard this is yet another useful feature that Apple have taken away. I'm comfortable on the command line, but I'm a little rusty, so I don't know how to manage the Recovery Partition. Would I have to mount the EFI partition on the external drive with Mojave and copy it over the EFI partition on my MBA? I'm not sure how I would do that, so any help would be appreciated. Right now, I think my Recovery Partition is from Mavericks because I had to do an emergency restore and the only Yosemite installer I could get at the time was a Hackintosh version. I think I'd like to just upgrade that to Yosemite Recovery Partition in case I want to do a restore from a Yosemite TM backup. 6) if I do format my MBA's internal SSD as an APFS drive, will the G5 be able to read the drive if it's shared over afp? I've heard a lot of people having problems with APFS and file sharing. I would seem to me that it shouldn't matter what format the drive is, as long as it's an afp share, any Mac should be able to read it. I just need to make sure my external drive stays HFS+ because it could end up inside older Macs sometimes. I know this is a lot of questions, but I'm really out of the loop with Apple ever since Jobs died and they got rid of Scott Forstall. It's just a mess that seems to keep getting worse! Thank you for any help you can offer! P.S. Someone already suggested I just clone my current drive to an external partition and install Mojave on my MBA, but I can't afford to be tied to an external drive for the week or so that it's going to take me to get all my software (tons of apps and 600+ audio plugins) tested and updated, so that's not really an option.