Seagate External HD

If its intended use is for Time Machine, then you will not be able to add any files to it. Time Machine wants the whole disk to itself, and will make it "read-only".

You may need to start from the beginning and describe which instructions you followed. For Time Machine, read Back up your Mac with Time Machine - Apple Support. Never use any non-Apple disk utilities to format or "manage" Mac disks. Seagate is fond of including those utilities with their hard disk drives. Don't use them.

You can try the free NTFS driver app from Seagate, its mentioned here.

if the drive had NTFS partition(s) (aka: file-structure) , then you just need to load new NTFS drivers from Seagate website, or obtain Paragon's new NTFS driver.

if the drive had APFS (apple's file-structure/system) then partitions/data inside it will not show-up inside a microsoft(MS) windows computer's Windows-Explorer (similar to apple's "Finder").

You will need either APFS driver in windows or another type of File-Explorer to read APFS inside Windows.

if the drive had NTFS (which is microsoft's file-structure/system) then partitions/data either will not show up in macOS in mac computer, or drive's data will show-up as READ-ONLY mode , you cannot write into the drive. But in MS-Windows, you can do read/write both in NTFS based drive.

Once you add new NTFS drivers in macOS, then macOS in mac computer will be able to access as READ+WRITE mode.

If the Seagate Drive is Formatted into the MS Widows Native Drive Format of NTFS format, the Apple Computer and see the drive and Read from the Drives.

The problem you appear to be having is Writing to that drives.

To able to Write to this Drive in the NTFS format will Require Special Software that needs to be installed on the Computer.

Then and only then can the User Write and Read to that drive

Seagate is fine. You just need to format the disk so that you can use it for a Mac.

Use Disk Utility. It is in your Mac's Utilities folder. Choose APFS as its Format.

Don't download or use any non-Apple utility to do that.

Janehc wrote:

Good Morning

I uploaded the required software when I initially bought the external hard drive and it worked. Now, a year later when I connected it to my Mac so I could load files on it, it will not let me. This is why I have no idea why it suddenly is not working when it did when I first bought it

Q - " when I initially bought the external hard drive and it worked "

A - That was Year ago and has the computer Software " to update " was used to control the Seagate Drive, was that upDated too.

A - Old Third Party Software used to control the External Drive needs updating too - no ?

Personally, I would follow the advise from @John Galt in above reply and get rid of Third Party Software that is often buggy at best and re-format the drive as per instruction.

Yes, macOS Software upDates and especially macOS UpGrades to newer versions of macOS tends to and often times Break Third Party Drive Management Software.

That would be reason enough, IMHO to follow the advise given by @ John Galt.

The Operating System is very well equipped to mange the drive without this extra layer of annoyance.

That being Third Party Software to manage external Drives

EDITED - spelling error

Yes. If you don't want to lose those files copy them to your Desktop (for example) instead. Perhaps create a new Folder on your Desktop in which to store them: use File > New Folder... and give it a name such as "Seagate contents".

To be absolutely certain you will be creating a copy of those files on your Mac, hold an Option key while you drag them from the Seagate into that folder. Otherwise you might just be creating an "alias" to the real files and you don't want to do that.

John Galt wrote:

To be absolutely certain you will be creating a copy of those files on your Mac, hold an Option key while you drag them from the Seagate into that folder. Otherwise you might just be creating an "alias" to the real files and you don't want to do that.

I am only 99.9% certain you can just drag them without having to hold an Option key. The above is an "abundance of caution" sort of thing.