Ill try and add as much info as I can.
I have an arduino nano and an SD card reader ( https://www.ebay.co.uk/itm/Micro-SD-TF-Memory-Card-Reader-Module-SPI-interface-For-Arduino-RaspberryPI/262460777856 )
I am using the PetitFS library to read ( I only do reads ) as I was getting too close to memory limits and was causing corruptions with the stack. The sd card is always inserted.
So, the SD card is conencted to a 5v line, which comes off a 7805 voltage regulator.
When the arduino is connected to the usb/computer the sd card inits fine, and works as expected.
When running from the external power supply ( 12v 1amp ) via the 7805, the sd card isnt read. however if i soft reset while powered, the sd card reads fine...
This suggests its a inrush power issue? I have tried to combat it by placing a 100uf cap between the vcc and gnd pins of the sd card - but this does not work.
Any suggestions? i'm loosing my hair!