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I use a Mac and an old HTC smartphone to read and write to a Micro SD card. Is the SD card definitely showing as a valid drive within Windows Explorer?

Most Android devices have an option to mount the SD card when connected through USB. Check the notification section when you connect initially as this is normally disabled by default. You should be shown an option to move from “Charge Only” to “Disk Use” or something along those lines.

From here, the SD card should show as a new drive (often with the name “NONAME”) and you should be free to use Win 32 Disk Imager. I use the dd command over Terminal to write images.