Disk Utility keeps telling me the partition has an issue and the disk is not writeable.
Also, windows says it is read only and I cannot format it on Windows 7 either. I tried Diskpart on windows and it just gave me errors. It seems I need to low level format it somehow to fix the partition table or find a way to force that operation.
MacBook Air
Two things jump to me.
1 - Some MicroSD drives have a Slider Lock ' Unlock on the side of the MicroSD card.
If in the Locked Position - neither Windows for macOS can do anything except read from the card.
2 - MicroSD Cards are useful but Not for " I was previously using it for time machine backups."
Regardless of how big of a Drive Capacity it may have, MicroSD cards are prone to failure and are very unreliable for TM Backup drive
It is suggested to get a reliable External Drive with at least 2 1/2 Greater Drive Capacity than the Internal Drive of the computer and use that drive for TM Backups
Refer to link below
Two things jump to me.
1 - Some MicroSD drives have a Slider Lock ' Unlock on the side of the MicroSD card.
If in the Locked Position - neither Windows for macOS can do anything except read from the card.
2 - MicroSD Cards are useful but Not for " I was previously using it for time machine backups."
Regardless of how big of a Drive Capacity it may have, MicroSD cards are prone to failure and are very unreliable for TM Backup drive
It is suggested to get a reliable External Drive with at least 2 1/2 Greater Drive Capacity than the Internal Drive of the computer and use that drive for TM Backups
Refer to link below
more info on the micro SD
ignore this reply edited above
FWIW, did you remove the card from Time Machine's preferences? When you assign a disk as a Time Machine drive, it is set to be read only. Plug the card in and see if it's removed from Time Machine.
Also, try erasing the whole physical drive. Within Disk Utility you may need to click "View" and select "Show All Devices" so that the physical drives appears on the left pane of Disk Utility.
How do I make my microSD card writeable so I can reformat it? I was previously using it for time machine backups.
