I have a Canon t1i. Tried using SanDisk Extreme 64GB (45MB/s) card and got an error message

[jsnsports]

What do I do to get the card to work in the camera?

[Skirball]

Have you formatted the card?  In the camera menu there's an option to format the card.  Try that first.

That said, the T1i is an older model and might not be compatible with SDXC cards. If it can only take SDHC it might be limited to 32 gb.

[ebiggs1]

Recording Media for T1i is:
SD memory card, SDHC memory card

I would never recommend a 64GB card anyway.  Just too many eggs in one basket for me.  I typically use 16GB, max, and use 8GB, too.

[Skirball]

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@ebiggs1 wrote:

I would never recommend a 64GB card anyway.  Just too many eggs in one basket for me.  I typically use 16GB, max, and use 8GB, too.

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The size of the card is irrelevant if you don't swap out cards in the middle of shooting.  A casual shooter could easily make a 8 gb card last for a 4-5 day trip.  Far more than that if they were shooting jpg.  Why not use 1 gb cards?  Most casual shooters don't swap out cards throughout the day, so the size of the card doesn't really matter.  Especially if you're DLing to a laptop at the end of the day.  Pros and anybody who has shot an "event" know better.