This problem is known and often reportet. It exists since Posidon-stack exists. It does'nt affect all type of sticks - only with slutty firmware-implantation (which are the most of the cheap ones). Makes no difference between useing Ambient or shell.

To avoid such problems you may split your Sys: to pieces of about 250-300MB and copy them to your stick.

Attention: Do not remove or start next action after Anbient or shell shows copying finished! The buffer is'nt written at this time. Wait about 15 sec. If you don't do that, at next copy action you get errors and nothing can be written. In this case put stick into an PC and delete or write something on it - this repairs it in most cases.

USB-HDs, CF or SD (with Adapter) does'nt show that behaviour.