Teensy 3.6 SD card writing program

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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!
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.
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The pins are not physically shared between the SD card and the SPI lines. Can there still be conflicts?
What is powering the 'custom radio receiver unit'? Does the SD access fail when the Radio is not in use?
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