You can change to another modern Mac model to format the 256GB SD card. It may work.
It seems that the 256GB SD card is not compatible with the 2009 MacBook Pro. Your Mac model is a little old and its hardware doesn't support cards large than 64GB. If you want to format the SD card, try to find a recent Mac model running on macOS Ventura/Monterey/Big Sur.
Here is how to format an SD card on Mac:
- Open Finder > Applications > Utilities folder, then launch Disk Utility.
- Select the card and click on the Erase button at the top toolbar. You might notice your SD card with a partition underneath in the left sidebar. Select the primary card that might bear the brand name, and then double-check that it is the card you wish to format by looking at its compatibility and available space.
- Assign a name to the card on the pop-up window.
- Select a file system from the drop-down menu.
- Keep the Scheme unchanged.
- Click Security Options to format the SD in a secure manner. Select how many times to write over the deleted data by dragging the slider, then click OK.
- Click Erase to start the process.
- Wait until the formatting process is done.
If Disk Utility fails to erase the SD card, you can continue to try Terminal commands. For more detailed steps, read the following post:
Read more: 3 Ways to Format an SD Card to FAT32 on Mac