G'day Derry, welcome to GPSPower.

[Only registered and activated users can see links. ] there's an explanation of the reasons why many older Garmin units can't use large [2GiB or more] map images as said by Fa_Piter, and it also explains the interrelationship of that with FAT16 and FAT32 files size limits even in spite of units such as your nuvi 660 having the ability to use FAT32 SD cards. Additionally, it explains why a maximum-sized FAT16 image [2GiB-1byte] wont fit on a so-called 2GB SD card.

The final para of that post explains that the FAT32 SD file size for some units such as US/EU nuvi 2x0/7x0 has been increased to >4GiB by a firmware upgrade even though those units have internal memories formatted in FAT16. This gave rise to some [albeit now fading] expectation that Garmin will do a similar fw upgrade for other older units like n3x0/6x0 but i wouldn't bet on it. I think the most hope would be if one of our resident GPS Software Experts has the inclination and time to isolate the changes made to other firmwares like nuvi 2 & 7 series and apply those to make custom fw for other older units. The same could apply to 2 & 7 series units now recognising a 2nd map as gmapsup1.img in SD:\Garmin folder by firmware upgrade being applied to 3 & 6 series.