"Time Machine couldn't complete the backup"

I got a Time Machine error message:

"The backup disk needs 1.05 TB for the backup but only 749.58 is available."

Last successful backup was back in July. I added a lot of files since then to this mac.

My backup drive is showing two icons on the desktop. One is "Backup" (official time machine icon, 750 GB free) and the other is "LaCie Share" (600 GB free).

I assume this means that the backup drive is partitioned, and if the two partitions are combined, there will be enough free space for the backup.

Is the only way to combine the partitions to erase the backup drive and create a brand new time machine backup?

Thanks.