I bought a prepaid sim card and used it on my android, but i had a problem using it and would like to return it. Is it safe to do so? I'm wondering if it could now contain my personal information since I used it, kinda like a hard drive?
A SIM card is used to identify the device to the network. The SIM card contains the secret authentication credentials to identify itself to the device. If you are worried about the SIM card being misused for authenticating as yourself to the network, you'd want to disconnect the SIM card from your account and phone number before disposing/giving it away. Your network provider may have already done so when you follow through the process for changing SIM ownership, deactivating a SIM card, or when registering a new SIM to your phone number, but you might want to double check with your network operator.
This begs one and only one question:
- Did you manually store any data on the SIM card?
No? Then yes, it's perfectly fine to return.
Older devices, like feature phones, that didn't have a lot of built in storage or had incredibly slow storage would store text messages or phone numbers on the sim card to save what little space they had and keep your things easily transferable. Eventually they realized how dumb of an idea this was and stopped doing it. So unless you have forcibly saved contacts to your sim, or text messages, or your phone is dirt old, then there is very little to no data on the sim card other than what was already on there.
I would never pass on a sim card to anyone, especially with what we know from the Snowden leaks. The SIM card is serialised and you are tied to it for the life of the card. If the SIM card is ever used for malicious intent (or at least something the powers at be might deem interesting) it is tied to you.
The worst case would be if the SIM card ends up in an IED, there would be a link between you and the IED.