I have had this problem and it was a USB that was too full or otherwise a problem. Try loading just that design on a freshly formatted USB stick. Hope it helps.

Cannot help but very interesting

I have a 350E and this has never happen. But I only load a design or 2 at the most. Is is the right format? It's not jef* format since this machine will not read that .
I also have only saved one design to my machine, due you save every design, if so there maybe no memory left? I am just guessing , you could phone you Janome rep and ask them.

I don't have the 350E but do have the 300E. It doesn't take a USB but it takes a Compact Flash card. I was told not to go too large memory size as it slows down the loading. I have a lot on my card and it still loads.<br /><br />
<br /><br />
The design you had digitised was it in JEF format?<br /><br />
Did it fit within the perimeters of the Hoop?<br /><br />
Does this only happen with this one design?<br /><br />
If so it is either the design that is the problem or the USB stick not your machine.
Try putting it on another USB ( make sure it is Formatted for the machine first)

Your design might be too many stitches. Your machine can only handle about 60,000 stitches. Even if the design will fit inside the hoop -- it cannot have too many stitches.
As others have mentioned, make sure the design is in the correct format.