My Arduino board, powered by own 9V supply, collects data. I'd like to read out the data later. But when I connect Arduino to my PC via USB it reboots and all data is lost. How can I have access to the data if Arduino is not connected to PC all the time?
Robert125: How can I have access to the data if Arduino is not connected to PC all the time?
Store it not volatile memory like EEPROM or on an SD card.
You can also get an FTDI board and use two other pins for serial (Using the SoftSerial library) so you don't need to wake the sleeping bootloader.
Adafruit sells a data logging shield that saves your data to an SD card. https://www.adafruit.com/products/243