Failed to store two RFID readings

Good morning.

I 'm working on a project where I have an RFID reader and an SD card where I store the exact time which has passed the card.

The problem is that when passing a card guard , but to save the next I restart the Arduino since the SD fails.

Please help.

Thanks for your attention.

The problem is that...

No, the problem is that you failed to post your code. You also failed to define specifically which SD reader and which RFID reader you are using.