I have a MacBook Air running Yosemite.
I have an external SD drive that I save files to frequently.
Info on SD drive says it's READ-ONLY all of a sudden? I am the ONLY person that uses this drive and it worked literally minutes ago!
Some SD cards' firmware will lock the device to read-only if any write error occurs.
If this happens, the only solution is to copy all data off it & discard.
SD/flash is not worth the effort of trying to fix, once it starts to go wrong.
Things to try…
Double-check the physical lock has not been flipped by accident.
Reboot the computer & try again
Try it on another computer/device
Once you have all existing data copied from it, attempt to format.
If none of this is successful, throw it away.
I work with hundreds of (admittedly very cheap) SD cards for work, & this is our standard practise if they start to fail.
Never store any important data on SD without having a safety copy on some more reliable medium.
I'd even extend that to - if you have to use SD to carry data from one machine to another - home/work etc - use 2 cards & keep them manually in sync with your work.
All of my SD cards are suddenly giving this erroneous Read-Only message. It started wit the 10.10.3 update. Before that they worked fine. I have cards from different brands and sizes. The problem is with the MacOS X update having a bug. Older computers that do not have the 10.10.3 update do not have the problem.