copying photos library to external hard drive

Looks like it's formatted for Windows? If that's the case, whey doesn't the disk utility erase/reformat work? Is there another way to format it for Mac?

Thanks for your quick reply. It says it's plug-and-play on any Mac operating system 10.5 or higher, and I'm running El Capitain, 10.11.6, so I thought it would work if I just connected it. I will try formatting it to Mac OS Extended or APFS, as you suggest. Does it matter which one? I'm not doing anything fancy - just want to use the external drive like a big USB stick.

I see. I think because I'm running El Capitain, I don't get the APFS option in Disk Utility. I do get the OS Extended option in two choices (Journaled or Case sensitive, Journaled). I should choose one of those?

I ran the First Aid on the mac mini - no problems - but it won't let me run it on the external hard drive, if that's what you meant. Still cannot drag and drop the photos library to external hard drive. I'm nervous about updating the operating system on the Mac Mini to High Sierra, but maybe I shouldn't be? And would that even solve this problem with the external hard drive?

No, it's a brand new hard drive - nothing on it. But when I try to erase it, I keep getting the same error message. The details say it couldn't unmount disk, whatever that means. I get the same error message whether I try to erase it and format it as OS Extended (Journaled) or ExFat. It's a brand new external hard drive, and it says it's plug-and-play, so I'm not sure why this is so difficult.

Ok, I just tried to format it as OS Extended (Journaled) and I got an error message: "Erase process has failed"

I've tried it a few times, but the result is the same.

Any idea why that might happen? Maybe I need to update the operating system (which I fear doing on a seven year old mac mini)?

It's an SP Armor A60, USB Gen 1 portable Hard Drive with 1TB of memory. Says it works with Mac 10.5 and later, and I'm running 10.11.6, so I thought it would work.