I'll assume MyDrive recognizes your unit and displays the current map versions on your 400.
You'll need to speak directly with TT chat there in the UK to sort it. Have your unit's serial number at the ready.
Your 400 should have 8GB of memory and an SD card slot, so that wouldn't explain it.
I'm at work, so don't have the TT chat # here, but I'll bet dhn has it at hand if he pops up here soon.