Possible data corruption
Most probably there is an error in the filesystem of the microSD card, which causes Ubuntu to mount it as readonly (thus saving your data from further corruption).
To verify whether you have a case of data corruption, start
and check if you get any corruption messages as soon as you connect the microSD card.
What you should do in this case is to copy the files from the microSD card to your computer and format it again with the Disk Utility.
Then, copy the files back to the microSD card.
Filesystem requires administrator access
If you formatted the microSD card using the Disk Utility, you may have chosen the ext4 or other Linux filesystems. In this case, the user ID of the folder in the microSD card might be different from your own user ID.
To verify whether you have a case of wrong user ID, run
and try to copy any file onto the microSD card. If the copy still fails, then you have the data corruption issue (thus, see above). If the copy works, then it is a permission issue with user IDs. You can use this Nautilus file manager to change the ownership of the folders/files on the microSD card. There is an option to
Take Ownership in Nautilus.