This question has been one that I have been interested in. there are many posts about this on the site. I have a garmin colorado 300 and an Oregon 400t. So, I asked Garmin support two questions: 1. can garmin devices use both SD and SDHC cards? The answer is below:
"The SDHC cards are more common for cards larger than 4GB. You can find regular micro SD cards at a lower cost. There is not a difference in speed of how the unit access the data. Our devices will work with any card up to the 4GB size."
Question #2: is there a size limit for SD/SDHC card for garmin devices? The answer is below:
"the units were designed to only read up to 4GB cards. The device has to read through the card each time it is turned on. So there is a hardware limitation on the device itself. As our technology for the Outdoor handheld devices continues to grow. The units created in the future should have more memory capabilities."
What my question is now to the forums: there are some posts that folks are using much larger SD/SDHC cards, like 8gb or larger (which are all SDHC cards). so, are you sure that your unit is reading all that space or are you just wasting everything above 4gb (per the garmin support response above)?