I have recently have had issues with my external HD's becoming "read-only". Not sure why yet, but here is the fix.

DISKPART

Open the command prompt box "CMD" at Run

Type each of these commands one at a time in this order:
diskpart
list volume
select volume n (where n = the problem volume)
attributes disk clear readonly
attributes volume clear readonly

After each of the last two you should get a response that it was successful.

Your drive should now be both read and write capable.