I had the same problem. My drive was working fine then one day it was read only. I thought I would have to re-format. I researched a little and it said sometime it is caused by an error that is caused when disconnecting the drive from a PC.
Just the other day I needed several files from the drive so I connected it back to my PC. I noticed I couldn't save the file because it was also read only on the PC. I then selected all the files on the drive and removed the read only in properties.
When I connected the drive back to my iMac to my surprise it worked as read/write! So my drive is operational again!
Hope this works for your!