Portable hard drive

That is excellent advise from Marco.

To keep your photos safe you should first,

1. Copy them to your Mac Mini and check to see if you can open and view them.
2. Then it would be a good idea to copy them to a second external hard drive that is formatted Mac OS Extended (Journaled)

Then when you are double sure that the photos are safely and in two other places, reformat the windows formatted drive and either copy the photos back to it or use it for Time Machine.

It would be best to use the larger of the two external drives for Time Machine.

see > Back up your Mac with Time Machine - Apple Support

If you can only read from the external drive, then probably it is formatted in NTFS format which is usually used on Windows machines.

To check if this is the case, select your portable drive in Finder and invoke ⌘-i. An info window will appear showing, among other things, the used Format.

You have two options: reformat the drive using a fully Mac compatible format (as you said) or enable the NTFS writing via third party utilities which offer this feature.

To answer to your question, if you copy all the data on your Mac and reformat the drive in a Mac-friendly format – like Mac OS Extended (Journaled) – then you can copy back the data again to the external drive and write other files in the future as well.

Marco's diagnosis is almost certainly spot on. Since the world is mostly a Windows world, external drive manufacturers tend to pre-format their drives for Windows - NTFS which can be read by the Mac but not written to. Marco's two solutions are also correct. Using a third party driver to enable NTFS is doable, I used such a driver for years but there is one drawback. There's no guarantee that an Apple update or upgrade won't break the driver and then you'll have to wait for the vendor to update the driver.

It is this reason that I recommend the second option: reformatting the drive. den.thed's suggestion of backing up to another external drive is also a good idea. And I'll quickly put in a plug for making sure that you've set up TimeMachine to back up your computer. If you have, and if there's enough space, you could temporarily back up your photos to that drive. Then format the pictures drive and copy the pictures back. Once you are certain all is well, you can delete the pictures from the TimeMachine drive. Finally, go into the TM preferences and add the pictures external drive to the backup list. You absolutely want your photos backed up to at least one hard drive.

Personally, I would get a second hard drive and format it properly,

copy over all the photos from the NTFS drive, properly reformat

the NTFS drive, then copy the photos back and from then on

maintain two backups of your photos. For something that

like photos that cannot be replaced if lost, one backup is no backup,

as backups can fail.

I maintain two complete backups of all my photo work.