I put a brand new SanDisk SDHC 4 GB memory card in and it says "memory card error; card is locked".

When your camera was made for SD cards (you did not help us here by entering a brand and model) it could be it won't accept the newer SDHC cards or the SDXC cards.
The SD cards were made up to 4 GB, but now even the 1 GB cards already are SDHC, because they are faster.
Again, when a device is made for SD cards and it did not get an software update, you most likely can't use SDHC cards and certainly no SDXC cards (above 32 GB)
And every card you put in your camera, first format the card in the camera, before using it.

SD and SDHC cards have a slide switch along one edge. The position farthest from the metal contacts locks the card, protecting it from writes. The position nearest the contacts unlocks the card.

The 4 GB memory card is not compatible with the camera. It's a SDHC card (high capacity) and the A520 only supports SD cards. (The difference between the SD and the SDHC is similar to that of single layer and dual layer optical discs. Not all drives can handle both formats.)

Documentation for the camera is here: http://usa.canon.com/cusa/support/consumer/digital_cameras/powershot_a_series/powershot_a520#BrochuresAndManuals .

I wish that I had better news for you.

Cindy Wells

The SD630 works with MMC and SD cards, 2GB or smaller. It does not work with higher-capacity SDHC cards.

Your SDHC card can be used in any camera that accepts SDHC cards.

The problem is with the card, it may have been locked here is a diagram to show you the slide switch
Image here

Also each time you use the memory card it should be formatted, Format all new memory card to the camera it will be used in and each time you download the pictures to a computer format the memory card to provide a clean medium for additional picture files

Your camera can only use SD cards up to 2GB....not SDHC cards.

The Z740 uses SD cards, not SDHC cards. SDHC cards will physically fit into the card slot but are not compatible. It's a hardware incompatibility, not updatable in firmware. You'll have to stay with 2GB and smaller SD cards.

No, 8 GB is too large. According to page 5 of the manual, 4 GB is maximum:

•When the term card is used in these
operating instructions this refers to the
following types of Memory Cards.
– SD Memory Card (8 MB to 2 GB)
– SDHC Memory Card (4 GB)
–MultiMediaCard (still pictures only)
More specific information about memory
cards that can be used with this unit.
• You can only use an SDHC Memory Card
if using cards with 4 GB or more capacity.
• A 4 GB Memory Card without an SDHC
logo is not based on the SD Standard and
will not work in this product.

The manual can be had below:

http://service.us.panasonic.com/OPERMANPDF/DMCFX33.pdf

it should have a slide switch on the outer casing of the card... write protection lock.

Has this card EVER worked with your camera? Since you didn't mention the model number of the Canon, it's possible that you have a compatiblity issue here. Most 4GB SD cards these days are actually SDHC cards, which store memory differently (in sectors, like a hard disc drive, rather than bytes, like your main memory). All SDHC compatible cameras also support SD cards, but cameras sold before the SDHC standard was created will not support SDHC memory cards.

There's a chance that Canon has a firmware upgrade to solve this. You would have to look on the Canon USA web site for this, based on the model number of your camera.