I now plugged in my eSATA drive. It shows up in /var/log/messages but you have to look for it, as dmesg will be flooded again instantly. According to
fdisk -lthe eSATA drive is on /dev/sdc and not sda which is mentioned in dmesg.
I then plugged in another external drive on the USB port: it appears as /dev/sdd and again it is instantly hidden in dmesg. When I remove the eSATA drive and plug in the USB hard disk on the combined eSATA/USB port, it is recognised as /dev/sdc. Same thing happens when the drive is plugged in at boot.
So I don't think a drive on the eSATA port will solve this issue.
Has anyone tested what happens, when you insert a memory card?