Unable to copy files onto backup disk

Hello everyone. I have a time machine which is used to back up and save all my computer data every two to three hours. It is a Seagate External Drive running with Windows Parallel 7 in my MacBook. It has been working quite well but just suddenly it stopped working. An error message appeared saying, “Files cannot be copied onto backup disk because it appears to be read only”. I have tried checking the time machine and when I chose a disk I cannot find any device. I don’t know what is happening here. It used to work well with me. Please help me by sending me your suggestions. Thanks a lot.
