A few more details... I used kernel-6182 & initrd from:
http://ftp.debian.org/debian/dists/stretch/main/installer-armel/current/images/kirkwood/network-console/qnap/ts-21x/ verified with help from: http://ftp.debian.org/debian/dists/stretch/main/installer-armel/current/images/SHA256SUMS I had to restart a few times because my previous RAID setup was detected by D-I and I was afraid it would interfere with the new install (I wanted to change partition layout) The (multiple-minutes-long) flashing at the end of D-I was getting on my nerves (I saw it stuck at 66% for a long time) but eventually finished properly. -- Vincent Legoll