iMac only boots when just the USB boot disk is connected

Hi everyone,

I have an iMac with Catalina installed and updated.

Mac OS system is installed in an external SSD drive. I use the internal HD just for storage, and I also use 2 other HD USB drives for the same purpose.

Everything has worked perfectly for four years, but maybe after last Catalina update (I'm just guessing), when I restart the iMac it gets stuck in the "question mark" folder message before starting up.

The strange thing is that if I unplug the two storage HD usb drives and leave just the SSD macOS disk it boots without any problem. It never happened before.

I have tried all the suggestions I saw, regarding NVRAM reset, and also choosing again the boot drive both from macOS normal mode and from recovery mode. But even if I choose the SSD macOS disk again, it will only boot if it is the only drive connected through USB.

Please help me :)

Antonio