My passport for mac

Choisya wrote:

... It says I have used1.9 of2TB but I have only backed up once before?

What is the capacity of your Mac's startup drive?

If restarting your Mac doesn't clear this up, then I would simply use Disk Utility to erase the back up drive and then start a new Time Machine back up.

For guidance please see:

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

And: Use Time Machine to back up - Apple Support

True enough, but the OP's WD Passport has no space, just 0.1Gb left, so will not backup. So, the OP has to get a new external disk, or clear up space in it to back up again.

It's always better to save one's most important data manually in a separate external storage. The manual saving of important data in external drives, also creates a good habit.

Your WD external disk is practically full. 1.9 of 2 TB, so you have to delete some files there, say make it at least 500GB free, and then reconnect it to your Mac. It will see it then.

Normally, deleting files to make more space would be great advice.

But NOT on a Time machine backup drive. Time machine backup files are double-locked, and if you manage to wrestle some into the trash, they will be stuck there forever.

Time machine manages its own free space. DO NOT intervene manually.

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it sounds like your computer has about 1.9 GB of "stuff" to be saved. if that is the case, you will need a bigger Time machine backup drive for trouble-free operation. Recommended size is at least 2.5 times the size of what needs to be saved.