Security update for ext SSD

[Dik2]

Trying to install Security Update 10.6.3. Mid 2011 iMac 27" running High Sierra on a Thunderbolt SSD. It goes through the usual restart process but Software update keeps saying it is needed.

[Forrest]

Security Update 10.6.3 was released in 2010 - won't run on High Sierra

[Dik2]

Sorry, it's 2019-003 10.13.6
It comes up from Software Update when booted from SSD.

[Filliam H. Muffman]

Do you have the most recent firmware installed?

[Forrest]

I can't get Security Update 2019-003 10.13.6 installed on my Mac Mini either. I think this is a result of uninstalling McAfee Total Protection awhile ago - I need to press the power button to shut off my computer.

[Sarcany]

I think your boot vol needs to be APFS to install the latest HS and Mojave updates. Did you clone to that external drive? Possible you set it up as HFS/GUID?

[Dik2]

Format: APFS

[Sarcany]

After the update, do you have an OS X Install Data folder at the root of the drive?

You should see that if the update failed to install properly.

It's been my experience that it takes multiple attempts to install on about one in four Macs, leaving that folder behind.

...Have you tried manually installing the update?
https://support.apple.com/kb/DL2004?locale=en_US

[Dik2]

Just tried manual install, no joy.