I have never used a raspberry pi compute module 3 but looking at Compute Module 3 Launch! it is said that it's based on the Raspberry Pi 3 hardware so it should run Raspbian. Usually you address SD Cards with a device file /dev/mmcblk0
for the first SD Card. I don't know what you mean with using it for secondary storage so maybe you will also find /dev/mmcblk1
depending on the primary storage. This storages usually have partitions, e.g.:
/dev/mmcblk0 storage
/dev/mmcblk0p1 first partition on storage
/dev/mmcblk0p2 second partition on storage
If a partition has a filesystem, for example ext4
, you can mount it, e.g. the second partition with:
cm3 ~$ sudo mkdir /mnt/part2
cm3 ~$ sudo mount /dev/mmcblk0p2 /mnt/part2
Now you should have access to this partition and can use it as normal folder:
cm3 ~$ sudo bash -c "echo hello world > /mnt/part2/hello"
cm3 ~$ sudo cat /mnt/part2/hello
hello world