Hi
I have done similar on other board. Following steps may helpful
1. Boot from sdcard
2. In shell(uart or ssh terminal) execute lsblk
It will show mmcblk0(maybe flash), mmcblk1(sdcard?)
3. Plug usb thumb-drive and mount it
4. sudo dd if=/dev/{eMMC flash block. Maybe mmcblk0} xxxxxx
Use dd command to extract flash disk to img file.
5. Access img file in other linux pc.
You can use loop0 or img mount in host and can see inside.
Regards