I have just tried installing with the Netinst version in which the firmware files are included http://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/6.0.0/amd64/iso-cd/firmware-6.0.0-amd64-netinst.iso and in that case the installation DOES work.
There may be something else involved in this as well. On this specific Dell machine I have the strange behaviour that during installation my Dell hardware RAID5 array is seens as /dev/sdb. There is also a /dev/sda present but it seems to have nothing attached. However, after the reboot when booting from the array it suddenly shows up as /dev/sda with a blank /dev/sdb. This did not happen in the past but I must have done something wrong on the hardware side a while ago as it shows the same behaviour when installing Debian Etch 4.0r8. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org