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I installed Mojave on an external SSD for testing. It formatted the SSD to APFS. It is very slow to boot to the Apple Logo, about 2 minutes. All works fine once booted. Why so slow to boot? Before always booted up in < 30 secs.
2 minute seems to be a bit long, but the boot process always takes a little longer when 2 or more bootable partitions are involved. In this case, it's your internal and your external. If you boot to your internal while the bootable external is connected, it will take longer to boot too. You can also see this if your internal drive was partitioned into 2 separate bootable partitions. I generally use 2 bootable partitions on my internal drives when testing upgrades, but once their released I repartition them back to single partitions and they always boot faster afterwards.
Boot speed is also going to be influenced by the type of connection in use, i.e., Firewire, Thunderbolt, USB2, ESB3 etc., something you didn't provide.
Lanny, my external is running off a USB3 port (2012 Mini). I believe you may be on to something about two drives connected at the same time. My internal drive is an SDSD HFS formatted and with the external SSD APFS formatted there may be some interaction resulting in slow external boot.
Same problem here:
iMac 21" later 2017 8GB RAM
SSD + USB 3.1/C case
Boot times:
High Sierra = 25 seconds after power on, Apple logo shows fast
Mojave (APFS) = About 3 ~ 4 minutes, big delay on Apple logo; after appear, Mojave starts fast.
Same issue as described on my 2017 iMac 3GHz. I'm running T5 Samsung SSD drive as my primary bootable/operating system (with bootcamp to my internal mechanical HD). Once I upgraded to Mojave, and rebooted, a blue screen appeared and would not go away (I did not wait as long as four minutes as described above). I've erased my external drive and reinstalled Mojave per Apple support and nothing changed. The only way I can boot now is by pressing "option" on startup and manually selecting the external drive (even though it is already pointed to this drive in system preferences>startup disk.
Thoughts??
Exactly the same situation here.
IMac 27" end 2013
External SSD on USB3
Previously on Sierra and High Sierra the boot was very fast (~30 seconds).
Now on Mojave I have to wait ~2 minutes with more than one minute of blank screen after the chime.
I made some tests, but nothing seems to work.
I connected a second external SSD USB3 and created two HFS+ partitions.
I installed a fresh Mojave 10.14.1 on one partition that was automatically converted to APFS.
Booting from this partition had the same behavior (~2 3 minutes).
As someone suggested that the problem comes from the APFS, I used SuperDuper to copy the Mojave APFS partition to the HFS+ new one.
Once finished I got Mojave installed on HFS+.
I changed the starting boot in the preference pane, shut down and restarted.
Unfortunately, the iMac boot is still very slow on Mojave HFS+.
Looks like others have the same issue as in this link:Problem slow boot with apfs on ssd external
Run Etrecheck. It might reveal some hidden memory resident programs slowing it down. Copy/paste the results here if uncertain.
Any findings yet on this?
slow boot on external SSD with Mojave
