After someone pointed me in the right direction, I found this info on Tigerdirect.com and hope they don't mind if I pass it on. I for one have puzzled over the compatibility of different formats for my Garmin Nuvi 880. Maybe this will be useful to others:
"Secure Digital (SD) is a flash (non-volatile) memory card format developed by Matsushita, SanDisk, and Toshiba for use in portable devices. Today it is widely used in digital cameras, handheld computers, PDAs, mobile phones, GPS receivers, and video game consoles. Standard SD card capacities range from 8 MB to 4 GB and for high capacity SDHC cards 8 MB to 32 GB as of 2008.
Like other flash card technologies, most SD cards ship preformatted with the FAT or FAT 32 file systems. This file system allows the card to be accessed on virtually any host device with an SD reader. Also, standard maintenance utilities can be used to repair or retrieve corrupted data. However, because the card appears as a removable hard drive to the host system, the card can be reformatted on almost any supported by the operating system.
There are different speed grades available for SD & SDHC cards, measured with the same standard as Optical Drives (in multiples of 150 kB/s - 1x = 150 kB/s). Basic cards transfer data up to 6x. High-speed cards are made with higher data transfer rates like 66x (10 MB/s), and high-end cards have speeds of 150x or higher. Note that maximum read speed and maximum write speed may be different, with maximum write speed typically lower than maximum read speed. Some digital cameras require high-speed cards (write speed) to record video smoothly or capture multiple still photographs in rapid succession. The SD card specification 1.01 allows for a maximum speed of 66x. Higher speeds of up to 150x are defined by specification 1.1.
It is worth noting that defragmenting an SD provides no performance advantage, since flash memory is truly random-access (unlike a hard drive), so there is no performance gain.
The SD format has proven to be very popular. However, the higher capacity SDHC cards, are not compatible with standard SD card reading devices. Since SDHC format cards have the same physical shape and form factor as the standard SD format, care should be taken to avoid confusion for consumers. SDHC cards require an SDHC capable device firmware, generally not found with older devices."
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