If the data is binary byte data and the arrays are fixed size then the simple method would be to just write the binary data buffer direct to the file (not as strings like println) and you can then use read to retrieve it back to a buffer.
Edit: Example
#define len 256
unsigned char buffer[len]
File f;
Write
f = SPIFFS.open("/buffer", "w");
f.write(buffer, 256);
f.close();
Read
File f = SPIFFS.open("/buffer", "r");
f.read(buffer, 256);
f.close();