Booting Off External SSD

I wouldn't mind converting the external SSD to APFS (if I can do this without losing data).

Boot into Recovery, select the Drive, then check the Edit menu for "Convert to APFS." I imagine it will be grayed out because the drive may be using the wrong partition map (table).

Why, when I selected the external SSD did it not give me the option to format as APFS? It arrive as a ex-FAT format.

Most are either NTFS or ExFAT, but the partition table is Master Boot Record (MBR). You cannot put APFS, CoreStorage, or any Mac encryption on an MBR drive. That all requires a GUID Partition Table (GPT). Also, by default, Disk Utility only shows you the usable Volumes on the drive, not the entire drive. You have to choose "Show All Devices" from the View button in the Toolbar.

There, you can select the device and erase it completely, changing the Partition Map when you erase. You can pick a Partition Map and a Format.

In Disk Utility, change to Show All Devices, select the device (make, model, S/N) and look at what is set for the Partition Map.

Is there any way to change the the Mac OS Extended to APFS without wiping out the contents? That is what Apple did to my internal drive when it upgraded to APFS.

Not on a drive formatted with an MBR partition table. Your internal drive was formatted with a GUID partition table. While Windows can use GPT, most drive manufacturers do not use it when they format the drive at the factory. They use MBR, instead.

'Select GUID partition Table' [whatever this is - when / where do I find this?]

See above.

Barney-15E Said:

"Mojave can boot from HFS+, but you may be right that it is why it doesn't show up in Startup Disk."

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I stand corrected. Go Here: I Forgot about this install: The macOS installer applies a firmware upgrade

Found this: Can I use CCC to clone an APFS startup disk to another Mac? - FAQ - Carbon Copy Cloner. As it reads: "If you attempt to clone an APFS volume to a Macintosh that has not yet received the firmware upgrade from the macOS Installer, that Macintosh will not be able to boot from the APFS volume."

Lawrence Hammer Said:

"Booting Off External SSD: [...]But this drive is not recognized as a boot drive via Preferences/Startup Disk[...]I am running Mojave and the disk is formatted Mac OS Extended Journaled."

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Use APFS for the Filesystem:

Mojave runs off of APFS - how it is running off of MacOS Extended(Journalized), I do not know. But that is probably why it is not able to be booted off of.

When I formatted the SSD via Disk Utility, it did not give me the option of formatting as APFS.

Then you didn't change the partition table to GUID. That is why it doesn't show up.

In Disk Utility, you need to select "Show all devices" from the View button popup menu.

Select the SSD device from the list and Erase.

Select GUID partition Table.

Some people have had problems with installing Mojave on APFS, so you may need to choose Mac OS Extended (Journaled).

With a GUID partition table, the Mojave install should convert it to APFS, but you could just try erasing as APFS, then install.

Barney-15E Said:

"Thanks everyone. Not sure to make of all of this. I should have stated that this is a 2017 iMac.[...]"

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You are welcome!

For thought...

Use Another Computer:

Try transferring this data using another Mac. Or maybe plug this into a PC, seeing if this drive pops up - who knows? If it works, see where things can go from there.

Realized my idea was wrong. disregard post.

Mojave can boot from HFS+, but you may be right that it is why it doesn't show up in Startup Disk.