How to prevent Finder windows from automatically popping up when I attach a disk?
Apparently it is set in the HFS+ filesystem header. I found some technical detail here, including C source to modify it.


  The idea is to set word 2 of the finder info field in the HFS+ volume
  header. The Mac OS 9/X Finders know to open a window corresponding to
  the dirID stored in this field.


And there's an ancient app to set it here.

For a tool to read the volume header check out fileXray (non free.) Its manual shows the location of the relevant flag on page 170, it looks like :

# Open folder ID
         finderInfo[2] = 0


If you're also using these disks for backups you probably aren't going to want to mess with the filesystem header, certainly not with the outdated tools above.