Time Machine-Not enough space to backup

I just had my 1 TB HD boot drive from a Mac Pro Early 2008 copied over to a 1 TB SSHD boot drive in a Mac Pro Early 2009. The HD from the Mac Pro 2008 is not installed in the Mac Pro 2009.

Previously, my Time Machine back up of my 2008 HD was working fine keeping a full backup and many older backups, consisting of, I assume, just the changes each time a backup was made.

The new 2009 Mac Pro shows two drives when reviewing storage using "About This Mac": Macintosh SSHD (284.25 GB free) and Macintosh HD (278.07 GB free) as well as the 1 TB Backup drive with (285.32 GB free). Now Time Machine has only one backup and gives the error message "not enough space to back up" every time Time machine tries to do a backup.

I need some ideas on how to troubleshoot and fix this problem.

Current Computer Config:

OS X Yosemite 10.10.5

Mac Pro Early 2009

Processor: 2 x 2.93 Ghz Quad Core Intel Xeon

Memory: 16 GB 1066 Mhz DDR3 ECC

Startup Disk: Macintosh SSHD

Graphics: ATI Radeon HD 4870 512 B