Greg and All:
SanDisk makes a LEVEL 10 card that reads/writes at 30 Megabytes per second now. It's called the "SanDisk Extreme" They come in 4 GB size and 8 GB size. I think but am not sure that they may go up to 16 and even 32 GB in size.
Level 10 refers to the speed that the data is written to the card. Faster is always better when it comes to writing data to the card and reading data from the card. 30 MB/sec is darn fast! You won't see much of a slow down using this card. You can save a waypoint and screen shot in about 1 second. Movies are recorded faster too. I think that this type of card writes about 200 times faster than the regular SDHC type cards.
The capacity is the amount of data that the card can hold. 4GB, 8GB, 16GB and 32GB refers to the capacity of the data card. GB = GigaBytes of data able to be stored on the card.
I found these SanDisk Extreme 8GB Level 10 cards at Best Buy in Evansville, IN.
But, I would guess that other Best Buy Stores carry these as well.
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4. That has been normal for the screen to slow down when making a sonar recording. What you need is a fester memory card and not a larger memory card. We recommend using the SanDisk Extreme III memory cards for sonar recordings due to their faster read/write times.