Time Machine Not Enough Space

Hello.

I have a Seagate External Hard Drive I use(d) as a Time Machine to back up my iMac. It was working just fine until a few months ago, when it started saying randomly "Backup failed: Not enough space on Seagate Backup Plus Drive"

It's weird to me, since it's a 2 TB External however my iMac only has 500 GB's, so there's definitely enough storage.

I thought if I emptied the disk completely, that would help, but it's still saying there's not enough space.

There's definitely enough space for the EHD, so I have no idea what is going on.

Please someone help me.