Tomtom510 when i turn on my tomtom 510 it wants me to install the sd memory card when i do that it shows the card to be pulled out with a big red cross on it please help

If your TomTom shows a red cross, it means the device is not able to load the software to be able to operate. Some devices have this software on a SD-card, some have it stored on an internal memory.
If there was a SD-card in your TomTom, I would suggest to put it back. If there is a empty SD-slot on your device, it could be you lost your SD-card, but some TomTom devices have an internal memory AND a SD-slot for memory extension, so if you have an empty cardslot it doesn't necessarily mean there has to be a card in it.

When you never saw a SD-card, probably some data went corrupt on the internal memory. When you have a SD-card, probably some data got corrupted on the SD-card.

To repair the software, you need to connect your GPS to your PC or Mac with a USB cable.
You also need the program TomTom HOME. You can download it for free on the TomTom website.
With TomTom HOME you can update the operating system of your device for free (map updates are nor free).

Try updating, and see if your device works again.

If this doesn't work, post it here.

That's a basic SD compatibility issue. Can you identify the maker of the cards, please? Got backup?

This problem is due to the memory card not containing the TomTom application or map. To update the application onto the memory card, Connect the device to your computer. Open the TomTom Home application. Click on 'Update My Device'. This should then install the application onto the SD card. Alternativly, you could perform all the steps above without the SD card in the device, this will install the application onto the internal memory. Note: you may also need to install your maps, voices, etc after you updat.

This problem is due to the memory card not containing the TomTom application or map. To update the application onto the memory card, Connect the device to your computer. Open the TomTom Home application. Click on 'Update My Device'. This should then install the application onto the SD card. Alternativly, you could perform all the steps above without the SD card in the device, this will install the application onto the internal memory. Note: you may also need to install your maps, voices, etc after you update.

The OneXL can't read form internal memory and SD card at the same time.. much like a floppy disk on a computer. If it shall boot from SD card, it need the software installed as well...

Red cross issue:
http://www.tomtom.com/support/index.php?Language=1&query=red+cross&FID=1898

Red cross issue:
http://www.tomtom.com/support/index.php?Language=1&query=red+cross&FID=1898

Device not recognised by PC:
http://www.tomtom.com/support/index.php?Language=1&query=connect+to+home&FID=7375

Device not recognised by TomTom Home:
http://www.tomtom.com/support/index.php?Language=1&query=connect+to+home&FID=7375

no that will not work see pullin the sd card out of the tomtom kinda pisses off the application if u have the red x error first do the soft reset if that does not work then update ur tomtom an application should pull thru as a download if not dont fret on tomtom home go to show contents on my one then delete just the application and i mean just the application then go back and hit update my one it will download and install new app and boom baby all done

Sounds like the memory card may be inserted incorrectly or corrupt/not correct for you model.

Try the clear flash utility. It worked for me. Martin