From what you’ve described, the issue isn’t the physical lock switch (you’ve already checked that) but rather the way the card is formatted or flagged. Some SD cards, especially those sold pre-formatted, come with write-protection at the partition level. macOS will then mount them as read-only, which explains why Finder shows "You can only read" with no option to unlock.
Here’s what I’d suggest you try:
- Double-check the lock switch on the side of the SD card—sometimes it’s just slightly off, and the Mac’s reader can be sensitive.
- Open Disk Utility again, but this time look at the left sidebar carefully. If the card appears twice (once as the card itself and once as a volume under it), select the top-level entry (the one showing the capacity, e.g. "31.9 GB Emtec"). From there, try Erase. Format it as MS-DOS (FAT) with Master Boot Record (MBR) scheme—this is the most compatible setup for car stereos.
- If Disk Utility still won’t let you erase or reformat, the card may be factory write-protected or failing. In that case, testing it with another computer (a Windows PC, for example) will confirm if it’s actually locked down.
Once it’s writable, you’ll just drag your music files onto it like any other drive. If you’re using iTunes/Apple Music, you can also select tracks, right-click, and “Show in Finder” to copy the raw MP3/AAC files over. If the card still refuses to allow writes after all this, it’s likely a dud SD card rather than anything wrong with your MacBook Air.
