Q: Seagate Backup Plus erase failure: Couldn't unmount disc

Hi CBiMac,

I apologize, I'm a bit unclear on your exact configuration or what you are seeing, but if you are having issues erasing or reformatting an external hard drive due to a "couldn't unmount" error, you may want to try booting to the Recovery partition and using the version of Disk Utility included there to try and do the erase/reformat. You may find the information and steps outlined in the following articles helpful:

OS X: About OS X Recovery - Apple Support

Resolve startup issues and perform disk maintenance with Disk Utility and fsck - Apple Support

Note: If you do so, make sure you are selecting the correct drive before you delete/reformat, as it is possible to select your normal boot drive or volume when booted to the Recovery partition. 

Cheers