seagate external hard drive snapshot option

I did a lot of tests with the Apple support, they even sent me a special software called Capture Data to analyse the logs, but at the end they told me 'we don't support this kind of issue with external drives'! I've tried 2 different USB3 boxes with different cables. This behaviour can cause some troubles especially every time I update macOS and the machine restarts, cause the installation doesn't properly come to an and, and I need to switch off/on to complete the process.

after some updates, the HDD is not used as the boot one (so the iMac starts with the internal HD) even if it's correctly set in the Preferences and also it's not listed if I press 'option'! I need to switch off and on the iMac or the hard drive enclosure to see it. I mean that it works perfectly if the iMac is switched off and I do a cold startup but if I need to restart it, eg. Since High Sierra and now up to Mojave 10.14.5, I can't boot an iMac 5K 27' late-2015 using an external HD Seagate Barracuda 2TB after a restart. I've read many forums to find an answer to this problem, but even the Apple support couldn't fix it, I hope to find some good suggestions here!

Seagate external hard drive snapshot option