When the sandisk is plugged into the computer, open My Computer via the start menu, and make sure first of all that your computer is reading that there is another drive there. Depending how many drives you have on yur computer as to what letter it will come up with. Mine comes up with J: as my adaptor has 5 drives on it.
Double click that drive, and a file will come up, and if you double click that file, thereis another, cant think what its called but it has about 4 letters. All yuor embroiderys must go into this file when downloading from the computer onto your sandisk. If they are not in this second file, the 300e will not see them.
Also make sure te sandisk is pugged into the adaptor properly, and then theat teh adaptor is pluggedin properly to the machine
is there a format option on your 300e for your sandisk? If so, reformat on the 300e. It does look like it doesnt do anything, as nothing will come up it just sits there.
Let me know how you go, and what you see if you try these out,