Error (-69879) - can't open disk

In Disk Utility, choose View => Show all devices. You can do this from a pull down menu in the upper left part of the Disk Utility window, or via the View menu in Disk Utility.

Then for the device you want to erase, select the highest level of the device, it should be listed under the External listing. Make sure you are selecting the correct disk. To be safe, eject all other drives, and double check that the device you are about to erase is listed in the External list. After selecting the highest level of the device, click Erase and specify GUID HFS+ for the drive format.

Victoria Herring wrote:

Thank you -- oddly enough the drive is mounted on the desktop and I have been copying things off of it in preparation to following your instructions == probably will do it tonight. I did think I had formatted it properly for Mac - and since there were contents on it, figured maybe I did = but it is now ExFAT -- thanks for the info ... it FINALLY mounted on the desktop - it is there but DiskUtility still says it's an ExFAT drive - but I can see the contents [am going to move them off, just in case.

When there is a long delay for a drive to mount but it eventually does mount, that can be an indication that the drive or its file system is damaged as the Mac may automatically run Disk Utility (fsck) in the background to try to repair it, which can take a long time for a large drive or for a damaged drive. When this happens, the drive is locked up with the fsck tool running and will not allow you to manually erase it.

It is hard to imagine how a drive can change itself spontaneously to ExFAT from some other format. The drive is somewhat suspect in my opinion. If you do manage to erase/format it, I would only put files on it that you have reliable copies of elsewhere, at least for a few months. You should have duplicates or backups of all important files anyway.