At startup warning comes on 10-30 seconds and then boots ok?

[bargbill]

Occasionally I get a European road sign, at startup you know the circle with a do not go here slash through it.
I have never seen this on a Mac.
Currently using Mojave 10.14
The warning goes away after a several seconds and then starts up just fine.
I Mac 27 inch with fusion drive 2gb and two external drives an 32Gb ram.
One drive is time machine, and other is saved work in progress.

 

[honestone]

Before you upgraded to Mojave, what Mac OS were you running? If it was High Sierra, that OS did not support Fusion Drives very well (regarding APFS). Mojave supposedly fixes that issue. Maybe that is not exactly "right" yet.

Also, how did you upgrade, "in place" (ie, basically just let Mojave overwrite what was on your machine already, or a clean, fresh, "virgin" installation?

Did you insure that all the third-party (ie, non-Apple) software you use is compatible with Mojave (maybe some of them have updates)?

Finally, have you ever done any disk cleanup/maintenance/repairs, from a software perspective? You actually can do a good amount of disk cleanup on your own, and there are some excellent programs available (both free and commercial) that can help with those tasks.

 

[bargbill]

Nice clean answer.
My system was High Sierra and then I went each and every beta Mojave update.
My current 10.14 was installed onto the internal, freshly formatted, and then data and apps were replaced on from TimeMachine.
Disk First aid was run on all drives before and after the two installs mentioned above.
Some errors were reported and repaired.

Is the euro road sign symbol a Fusion Drive warning specifically?

 

[honestone]

Not sure about the road sign and Fusion Drives, but that could be a possibility, especially given that things were fine with High Sierra. And I surmise this issue did not arise when you were running the beta versions of Mojave. So something happened with the final release.

OS 10.14.1 is currently in beta testing, but nothing was mentioned about Fusion Drives.