Using external drive as boot on new iMac

[LLA]

Hi,

I just bought a new iMac and instead of using the fusion drive to boot and run applications/have "home," I want to use an SSD drive that I have in an enclosure and connected to the iMac. I am moving from a mid-2010 MacPro, where I had this same SSD as the boot drive, and the OS is High Sierra (as far as I could update that computer). Now...

I have called Apple tech support and we tried to boot from that drive, but it just gave me the circle with a line through it. No go. That is literally as far as Apple was willing to go before sending the issue to the engineers. It's been a week and no one is getting back to me.

I am certain this is doable, I spoke with Apple before buying the computer to make sure it was doable. I spoke with OWC where I bought the enclosure to make sure it was doable. I am guessing that the iMac isn't reading the older OS?? Maybe?

So, I ask - anyone have any idea why the iMac won't boot from that SSD? Thanks in advance for any insight!

 

[benshive]

If you still have the older Mac maybe boot into the drive there, backup all files you want to keep somewhere, and then use Disk Utility on the new Mac to wipe the drive and do a clean install of the OS.

 

[LLA]

If you still have the older Mac maybe boot into the drive there, backup all files you want to keep somewhere, and then use Disk Utility on the new Mac to wipe the drive and do a clean install of the OS.

Well, I can see the drive on the new Mac, I just can't boot from it. So I guess I can just back up there. And then how do I properly get the content (all the users folders that have current content, music and photo library, etc. Basically everything that's on the drive) back to that SSD after it's been wiped?

 

[smirk]

Your new Mac is newer than the High Sierra installation, perhaps that's why it won't boot; the OS doesn't have support for the newer machine. If you're ok with updating the OS, connect the external drive to the iMac and upgrade the OS on it, then you should be able to boot into it. If you upgrade the OS instead of wiping/installing, all your data will be preserved.

But, of course, always make a backup first.

 

[LLA]

Your new Mac is newer than the High Sierra installation, perhaps that's why it won't boot; the OS doesn't have support for the newer machine. If you're ok with updating the OS, connect the external drive to the iMac and upgrade the OS on it, then you should be able to boot into it. If you upgrade the OS instead of wiping/installing, all your data will be preserved.

But, of course, always make a backup first.

Thanks. I think you're right. What is the proper way to update (without erasing the drive) the OS?

 

[smirk]

Just download the Mojave installer and run it (when booted from the iMac's internal drive), then when it asks you which drive to install on, choose the external.

 

[Fishrrman]

YES, you can boot and run a 2019 iMac from an external SSD.

HOWEVER... I don't believe a 2019 iMac can be booted from High Sierra.
SSD or not, won't matter.
You MUST have the proper version of Mojave to boot a 2019 iMac.

Other factors:
- what kind of enclosure are you using (USB3 or other)?
- what format is the drive currently in (HFS+ or APFS)?

Pathway to a solution:
1. BACK UP the data on the SSD to another drive.
2. Connect the SSD to the new iMac.
3. ERASE IT using Disk Utility to HFS+.
4. Install a clean copy of Mojave onto it (drive will be converted to APFS during install). (see alternative below)
5. Once done, restore your data to it.

Alternative approach:
1. Connect drive to new iMac.
2. Again, erase it, but this time to APFS
3. Use CarbonCopyCloner to clone the OS from your iMac to the SSD.
4. Now restore your data from your backup.

 

[smirk]

The way I read it, @LLA has already been using this drive as a boot drive on a different computer, and wants to now use it as the boot drive on the iMac. In other words, he/she wants to preserve the data on the USB drive. So I see no reason to format the drive, install Mojave, and then copy back the data, that's just a lot of work with no benefit other than possibly fixing a corrupt installation, which wasn't mentioned.

Just upgrade to Mojave in-place and you'll be fine. But, again, back up your data first just in case.

 

[LLA]

Just download the Mojave installer and run it, then when it asks you which drive to install on, choose the external.

Can't seem to find a stand alone version.

The way I read it, @LLA has already been using this drive as a boot drive on a different computer, and wants to now use it as the boot drive on the iMac. In other words, he/she wants to preserve the data on the USB drive. So I see no reason to format the drive, install Mojave, and then copy back the data, that's just a lot of work with no benefit other than possibly fixing a corrupt installation, which wasn't mentioned.

Just upgrade to Mojave in-place and you'll be fine. But, again, back up your data first just in case.

YEP! I did that last night and all is well. Just downloaded the installer and then installed on the SSD (without erasing) and it worked perfectly. A lot easier of a process than I thought was needed. Thanks @smirk !