WD My Passport Ultra Failing to Back-up (Error says it's read-only)

Have had no problems for the past year and a half, but suddenly got WD My Passport Ultra error message on trying to backup:
“Files can’t be copied onto the backup disk because it is read-only. You may need to repair or erase the disk using Disk Utility. If the disk can’t be repaired, select a different disk for backups.”
Tried Disk Utility twice and says it failed.
What now?!
Don’t want to lose existing backups.
Using with MacBook Pro with OSX El Capitan

Hello jbee,

You should try contacting WD’s Technical Support about this.

To Contact WD for Technical Support