Teensy 3.6 SD card writing program

[BINIT]

I am using Teensy 3.6 connected to a custom radio receiver unit. I have a transmitter unit which is transmitting data. The program works fine when I am using the serial monitor. However, everything locks down when I am writing data to an SD card. The command SD.open is returning false every time I am using it. The SD card writing function works fine if I use it separately with another program. I am writing a string and have tested it int but nothing works. I am using the SD.h library. Is it a timing issue or a hardware issue? Please let me what you think. thanks!

[JBeale]

Does the radio also use SPI? Is there any chance of a SPI bus conflict between the custom radio receiver unit and the SD card?
EDIT: nevermind, assuming you are using the built-in SD card socket on the T3.6 using 4-bit SDIO that is separate from the regular SPI bus.
However, since the libraries date back to earlier Teensy models which generally used the regular 1-bit SPI port to connect to a SD card, there is still a chance of (for example) having the wrong pin select set in software.

[BINIT]

The pins are not physically shared between the SD card and the SPI lines. Can there still be conflicts?

[WMXZ]

If you don't show us the source code, we cannot tell.

[defragster]

What is powering the 'custom radio receiver unit'? Does the SD access fail when the Radio is not in use?

[BINIT]

How do you think I can test that?

[BINIT]

Do I have to check that in the library file?