You are not alone in having the SD card not mounting issues with the macOS Monterey version. When this problem occurs, you can try to use an SD card reader, cold boot the iMac with/without the SD card inserted, update macOS, disconnect all peripherals and rest NVRAM, etc., to get the SD card mounted on your Mac.
Many other Mac users reported that their SD cards are not mounting on Macs, some experience this problem intermittently. According to their descriptions, such an issue is related to the version of macOS Monterry, it may happen to certain versions and SD card brands. It is hard to identify the exact cause of this problem.
Even so, there are some methods that are proven to be working by Mac users when they encounter the SD card not mounting on Mac. You can try them out to mount the drive on your iMac so that you can read and write data to it.
- Use an SD card reader instead of the built-in SD card slot.
- Cool boot iMac with the SD card inserted, if it fails, disconnect the SD card, cool boot again, then reinsert the SD card.
- Disconnect all connected external devices, shut down your Mac wait a few minutes, then reset NVRAM.
- Update macOS to a newer version and the bug may be fixed.
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