when you remove the SD-CARD and power on the wavedrum without it, you have this message error E1 display (if my memory don't failed)
maybe it's the SD-CARD memory have manufacturing defect ?, or sometimes the contact between the case card and the SD memory disconnect ...
...(like random micoscopic cuting signal on some pin memory depending when you make a small vibration on your unit when each time you pressed a button for example).
i have try to start the wavedrum with card inside, and after the boot finish and the sound preset choice done, i have remove the card, wavedrum still work because all nacessary are load in internal RAM memory, the E1 error appear only if i change the sound preset, cause when you changing preset request to load another pcm sound, algo and other things from the SD-CARD.
anyway, the best solution it's return your unit for repair in warranty.