Thanks all for your suggestions. It is a Marantz PMD 661 and not the newer model. When I've tried to format, I've left it for about 10 or 15 minutes and it still says "executing." I have not tried any other SD cards, or smaller ones, a good suggestion. I will also look for the firmware to update it and see if it will work with the larger disk then.
Best regards,
Laurie