Yes you initially formatted this as a windows drive, see master boot record, it is this that is preventing it from being partitiond again.

When you got the disk and it was only ever going to be used with a mac you needed to format it as Mac OS Extended (journaled) on a GUID Partition Map.

Sure.  A lot of that may depend on the specific implementation that Apple uses.  I think it may also be power limited.  It would require someone willing to make one.  Given how ubiquitous 2.5” drives are, I’m not surprised that there aren’t any bus powered enclosures for 3.5” drives.

Hi Turchee.

If Ak's suggestion doesn't resolve the issue, I would recommend that you get another external hard drive and transfer the contents of your original external drive to it. Once you have a backup, open Disk Utility and use First Aid to repair the permissions on your original external hard drive.

Howard

Ideally you would have things set up so that starting iTunes with the external offline doesn't cause you any problems, and you would have a backup of your iTunes library and other important data on another drive. Syncing your iPod has no impact on the files on your computer. You add songs to your iTunes library. Whether or not you then add them to the iPod won't change how much storage is being used on the internal and external drives.

Check under iTunes (Mac) or Edit (Win) > Preferences > Advanced and look at the location of the media folder. Is where you think it should be on the external drive or has it been reset to the internal drive? If it is wrong change it back to your preferred location, consolidate the library (File > Library > Consolidate Files), then delete the now redundant media folder on the internal drive.

tt2

Thanks to agodfrin in another post who said he  did a full reboot, everything worked after that.

And it also worked for me..

I was under the assumption that after the update and install of the new OS version, the Mac had restarted fully. Not so apparently.

I can go to bed now...

Hope this helps for you too

Disk Utility for macOS Sierra: Erase a volume using Disk Utility

Yes, however use the second HD to create a bootable clone using either SuperDuper! or CarbonCopyCloner. Much wiser than having 2 Time Machine backups.

Looks like a good plan.  I would make the step 5 the first one, however. Disable "Optimize storage" before you consolidate any referenced images. After consolidating they might become optimized, if Optimize storage is still enabled.

On her Mac she has chosen a year ago for optimizing the local photo library.) How can I check if this proces is ready?

That is not easy, because Photos does not have an indication, how many photos are still optimized. The process has not finished, if you try to disable iCloud Photo Library and get a warning, that photos have bot been downloaded in full. If you are seeing this warning, cancel the sign-out of iCloud Photo Library.

You can speed up the download by creating an album with all photos and videos an then enable "Download Originals to this Mac" for this album.  To enable this flag, select this album with all photos in the sidebar, then ctrl-click this album, and click "Download Originals to this Mac".

OK, me again!

It looks like App Store "remembered" my original El Capitan download for another machine, eons ago. I should have used the "Purchased" button in App Store. Looks like problem has been solved. Anyway, thanks to those you took the time to read my original post.

Paul

Sounds like it was corrupted from an improper shutdown.  I've had that happen when I forced a power down with my environment frozen.  It doesn't usually happen, but it can happen.

What do you need out of this?  Sometimes the only thing that can be done is to reformat and start over again.  If you have a backup, you can start from that.  If you have something important on that drive that's nowhere else, you might want to get whatever you can off that drive and then put it aside for when you can attempt to recover that data.  If it's not removable you've choices to make over how important that data is vs getting your machine running again.