When calling FATEngineStart , the fifth parameter affects the readOnly setting. Set to anything other than 0 and you can't write to the card. 

fat.FATEngineStart(_SD_DO, _SD_CLK, _SD_DI, _SD_CS, 0)
fat.mountPartition(0)
fat.newFile(string("test.txt"))
fat.closeFile()
fat.openFile(string("test.txt"),"w")
fat.writeString(string("this is file test.txt"))
fat.closeFile()

fat.openFile(string("test.txt"),"r")
fat.readData(@buffer,fat.fileSize())
debug(zstr(@buffer))

Refer to this forum post for object files: https://forums.parallax.com/discussion/175711/cant-get-the-fat32-object-to-create-a-new-file

The Parallax Downloads page includes the Propeller 1 Application Notes. Search for AN0 to get all the available articles.

This is the direct link: https://www.parallax.com/download/?search=AN0 

The Propeller family of microcontrollers are multi-core realtime processors, designed to perform multiple tasks simultaneously and deterministically, without the need for interrupts or the dictates of an onboard operating system.

The Propeller 1 (P8x32A) has been in production since 2006, while the Propeller 2 (P2X8C4M64P) is a 2020 release.

Click here for more details! 

• Join us at our monthly Propeller Live forum and ask questions directly to the Parallax engineering team. Check our Events page and click on the Propeller 1 & 2 tab for the latest dates.

• Join our friendly community forum where you can ask your Propeller 1 or Propeller 2 questions. The IDE development guys hang out on the forums too, so it's a great place to get their attention!

• E-mail us at support@parallax.com

Popular community supported IDE's include:

• SPIN Tools IDE : Supports SPIN/PASM & SPIN2/PASM2
• Visual Studio Code : Supports SPIN2/PASM2, BASIC, C
• FlexProp : Supports SPIN/PASM, SPIN2/PASM2, BASIC, C

Compiler (Typically included with the above IDE's)

• PNUT Compiler : Official SPIN2/PASM2 Compiler for the Propeller 2 

Refer to our Propeller Programming Tools Comparison Chart for more details.

We recommend our Propeller 2 Developer Starter Bundle, available from the Parallax Shop here:

https://www.parallax.com/product/propeller-2-developer-starter-bundle/