You didn't say if your unit was new or used. I checked their website (http://www.bikemp3.com/GL1800-Digita...GW-IPD-V03.htm
) and it looks like they had issues with some of their older units.
According to the website, the unit accepts SD cards up to 8GB. Folders must be numbered (1, 2, etc.) and contain up to 99 songs in each folder. The number of folders on the SD card will depend on the model of your bike.
I would check the formatting on the 256 MB cards vs. the larger capacity cards. Perhaps try reformatting one of the larger cards and putting only a couple of folders on it with your music (identical to the 256 MB cards).
If the larger cards still won't work, your unit may require a firmware upgrade to recognize the larger capacity cards. At any rate, it would be a good idea to contact the manufacturer regarding your problem. They will likely have a quick fix for you.