Random/Weird Garbage Recorded/Writings on my SD

Hello, my project logs data to an sd card every 15 seconds or so.
Sometimes things like this appear: Click on the attachment below.

I have an RTC, a Humidity sensor, temp and some other sensors. Everything is run on a mini pro 5v.
Also this Arduino connected to the sensors works in 5V, and it’s a slave connected to a master which operates from 3.2v to 4.2v. Do I need a I2C voltage step-down?

Let me know if you need more details to figure out why this happen. Thanks!

Something is going wrong for sure.

Did you format it with the special format tool ? http://forum.arduino.cc/index.php?topic=228201.0

Did you try another SD card ?

Are the 5V signals from the Arduino properly converted to 3.3V signals for the SD card ?

Perhaps the code is wrong, or you have not enough ram memory. Do you use many String objects ?

Peter_n: Something is going wrong for sure.

Did you format it with the special format tool ? http://forum.arduino.cc/index.php?topic=228201.0

Did you try another SD card ?

Are the 5V signals from the Arduino properly converted to 3.3V signals for the SD card ?

Perhaps the code is wrong, or you have not enough ram memory. Do you use many String objects ?

It is formatted correctly, The 5V signals? I assume you mean the power line and it does fluctuate a little when in use, the led's from some sensors flicker when the SD is writing, I think. Perhaps I need more juice on the power line? Also this arduino connected to the sensors works in 5V, and it's a slave connected to a master which operates from 3.2v to 4.2v. (forgot to mention :/) I use myFile.println(""); for every reading from each sensor. There are like 30 of those, does that affect memory Ram?

I'm a little confused how you have connected it and how it is powered. I had never 5V leds flicker when writing to SD card. You have to look into that.

Did you use that format tool in the link ? Did you try other SD cards ? You may not connect Arduino 5V signals directly to a SD card. Which module do you use ?

Could you measure the 5V pin. It should be 5.0V, if it is 4.5V then it is too low (in my opinion).

Many sensor work at 3.3V. Are they connected to the I2C as well ?

Yes, a Serial.println("Hello") uses ram. Use the 'F()' macro to avoid that.

  Serial.println("Hello");   // The text "Hello" is in rom and in ram !
  Serial.println(F("Hello"));  // The text "Hello" is only in rom

The other module could run at 3.2V and it is connected via I2C ? That makes me frown. How are the pullup resistors connected ? Current could flow into the pins of the 3.2V Arduino, or the 3.2V could not be recognized by the 5V Arduino. A level shifter avoid problems and makes it reliable.

Perhaps the I2C bus is working, and you have not enough ram. There could also be bugs in the sketch. Do you have a schematic and can you show the sketch ?