hi pirated,
Congratulations!! on getting piCore running on your RPi3B+.
You are not giving many details of what you are doing so we have to guess what step your are up to.
Usually on a Windows PC you will only see the first partition on the SD card.
On piCore, only the second partition is mounted. The first partition is used during booting but unmounted to make it more secure.
Follow the instructions in the link Rich posted. You don't need to worry about mounting or unmounting partitions.
m gives you fdisk help.
The greatest thing about doing this, is you can always burn a new piCore image in a minute and start again! It's a good opportunity to learn fdisk without a lot of pain.
second partition = /dev/mmcblk0p2
start sector = StartLBA
regards
Greg