Disk Utility can't repair external hard drive, tried fsck whats the next step?
If your disks have non-critical data you can live without, and you just want to wipe them clean, use Disk Utility. If you're not in a rush use the Secure Erase/Zero Fill option as this will take any bad sectors out of use that may have formed since you acquired the drives.

Alternatively, if you have data you really need to access, try DiskWarrior as it can repair many disk issues that Disk Utility/FSCK can't.