hi jbbum,
It might be better if you add your mount command in /opt/bootsync.sh. This gets backed up and will be persistent. /opt/bootsync.sh and /opt/bootlocal.sh are there for adding commands that need to run at startup.
Usually, it's best not to modify TinyCore/piCore commands.
I haven't used piCore for a while, but there was a stage where USB drives did automatically mount, via udev I think.
rebuildfstab runs during the boot process and also when you hot plug USB disks.
The mount command can be just $ mount /dev/$DEVNAME as the command uses the mount point found in /etc/fstab.
regards
Greg