I have a Mac Pro 5.1 (2010) with 3 HDDs installed.

One is a 1 TB running Snow Leopard. Another one with 2 TB for file storage and a third one I have added recently (WD Gold 4 TB).

I have tried to upgrade my OS to High Sierra, but firstly the new HDD didnt show up. Later on when I managed to get it displayed, I formatted it with Mac OS Extended (Journaled) and GUID Partition Table. I ended up with 2 Partitions. 1 TB for the OS and 3 TB for Files.

After the successful Installation, I booted up and a Prohibited Symbol popped up. I restarted and held down the alt key to select the same partition again.

Long story short. Every time I boot up the symbol pops up once and after a reboot, I can use the OS. The same process over and over again. Keeps repeating. One time it works, then not, then it works again.

I have downloaded the High Sierra installer and used the (Install Disk Creator) provided by MacDaddy.io

Everything works like a charm as soon I boot in, but it is really annoying that the first boot always get blocked with the prohibited symbol.

What is causing this weird issue?

*I already tried reinstalling OS in SafeMode.

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