While trying to install Debian 8.2 on a set of drives to be RAID1ed by
mdadm, partman wasn't listing either of the drives. Dropping to a shell
showed that both drives existed (/dev/sdb and /dev/sdc were present).
Eventually discovered that the drives had been part of an old mdadm array,
and mdadm was trying unsucessfully to assemble them. Consequently, both the
failed mdadm array and the two drives were being hidden from partman.
Manually stopping the failed array using mdadm --stop and rerunning Detect
disks caused partman to list the drives, partition them, RAID them, etc as
normal.