Um loop rápido e sujo fazendo isso:
#!/bin/bash
# get all disks into an array
disks=($(ls /dev/sd* | grep -v '[0-9]'))
# loop over disks in array
for i in $(echo ${disks[@]}); do
# check if it has a partition
diskcheck=$(ls ${i}* | grep '[0-9]')
# if not, print it
if [ -z "$diskcheck" ]; then
echo $i
fi
done