Whatever reason you had to use Ubuntu when creating your tinycore portables:
- Have you ever ended having a pendrive unable to load/install extensions?
- Did you find any problem related to permissions?
- Did you used a Debian related distro as the system host?
Then use GPARTED instead of the Disk Utility tool, which seems to be the cause of this issue.
Also, ensure to create first a partition table (at the Gparted menu) for that usb device.
Everything should be fine now.