Not a SIM card it is a SD card. Sounds like either the SD card is bad or there is a problem with your ability to read the card.

If it is a 4 GB SanDisk card, I have been seeing a lot of these cards in other devices that go bad and cannot be recovered even with specialized SD card formatting software.

Suggest you purchase or try a different SD card and make sure you are not having a problem reading the card from your specific machine.

Are you using a stand alone card reader?

Do you have a built in reader in the computer?

Are you using the USB cable and trying to read the card directly from the MaxiDAS?

You need at least a 8GB size Sandisk SD card now and a current update subscription if you expect to get anywhere. If you did not create a back up SD card you are SOL. Hopefully the card slot contacts and hardware inside your DS708 is still good.E-mail support with your unit's serial number and ask for instructions.

 After deleting the files on the card, you need to run PC Suit to download software in order to make it recognizable by DS708. Btw, yes,  you need to have at least 8G SD card.