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Currently I have a PI 3 B running with Raspbian Jessie. It has a 3 TB external Usb-HDD which holds the root partition for the Pi. The SD-Card is only used to start the Grub and afterwards the HDD takes over (which is much faster than running from SD). As the 100 MB ethernet port isn't fast enough, I added an external Usb 1 GB ethernet adapter. Currently the PI does routing and store files.
I then switched to the PI 3 B+, I simply put the SD of the PI 3 B into it and plugged all cables from the previous PI into it. But the PI 3 B+ doesn't seem to boot. The HDD doesn't fire up either. When I remove the SD card and boot again, the PI fires up the HDD but other than that nothing happens.
I also tested the PI 3 B+ with only the SD-Card (the one provided with it, which runs NOOBS), powersupply and connection to an HDMI monitor. It only shows the multicolored startscreen and nothing else, but it shows a flashing lightning symbol.
I have another PI 3 B that boots without problems like the one that I use as a router. I got all PIs from the same vendor.
Is there something special that the PI 3 B+ needs to boot ?