Pfaff Embroidery Machine 4.5

I have the above machine but am having problems saving design files to the USB stick. I have transferred the designs on to Premier Embroidery and followed the Export method. Some designs are OK but others look perfect on the PE page, number of stitches, colours etc., however I get the message that the USB stick is corrupted but when I insert it into the machine it's OK with the files that have been saved on to it. Where am I going wrong? I wait until it's safe to eject the USB, but I don't get a picture of an egg timer when the files are transferring.