Tip: Mac SDHC/SDXC card readers wrongly showing cards as read only

This is quite common on older Macs. The symptom is you see the usage of the card but not the free space in the Finder status bar. It’s due to poor tolerances in the drive, perhaps dirt build up as well, although using an air blower didn’t help.

What worked for me was positioning the write-protect tab slightly “north” of centre – i.e. almost at the standard read/write position, but not quite.