Ok ... so I've solved this problem myself ... unfortunately I can't really explain why this was a problem. So I observed thay my sda1 was owned by the user "rexroot" and only that user had access to the directory. So I unmounted the drive in the CtrlX UI and then I could see the /media/sda1 directory now belonged to "root" ... if I mounted it again, it changed to "rexroot". So I unmounted it, and reformatted it from the UI. After that was done and I mounted it again, it belonged to "root" and now I'm able to write data to the hard-drive. Could it be, that there was a bug in previous CtlrX OS versions, that set the username of the external media wrong? I know this was an issue I was having since the beginning.
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