Just check if the output of mount contains the drives you expect to be mounted.
- if grep is successfull -> OK, drive mounted
- if grep is not successfull -> drive is not mounted, now react appropriately
root@FB4040:~# mount
/dev/root on /rom type squashfs (ro,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,noatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,noatime)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
/dev/mtdblock14 on /overlay type jffs2 (rw,noatime)
overlayfs:/overlay on / type overlay (rw,noatime,lowerdir=/,upperdir=/overlay/upper,workdir=/overlay/work)
tmpfs on /dev type tmpfs (rw,nosuid,relatime,size=512k,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,mode=600,ptmxmode=000)
debugfs on /sys/kernel/debug type debugfs (rw,noatime)
/dev/sda2 on /mnt/sda2 type ext4 (rw,relatime,data=ordered)
/dev/sda3 on /mnt/sda3 type ext4 (rw,relatime,data=ordered)
root@FB4040:~# mount | grep '/dev/sd'
/dev/sda2 on /mnt/sda2 type ext4 (rw,relatime,data=ordered)
/dev/sda3 on /mnt/sda3 type ext4 (rw,relatime,data=ordered)
root@FB4040:~#