Time Machine claims not enough space, but there's plenty

Hello --

I'm having a problem with my Time Machine backup ever since upgrading to Yosemite. After the initial backup, I get these errors: "The backup disk needs 838.03 GB for the backup but only 712.05 GB are available. Select a larger backup disk or make the backup smaller by excluding files."

However, there is plenty of space. The initial backup took up about 898 GB of my 1.61 TB machine. Subsequent incremental backups should only take up a fraction — and I'm confident that I haven't added hundreds of gigabytes of new data to my disk. However, each time Time Machine tries to run its backup, I get the same error.

Time Machine is backing up both my small internal hard drive (128 GB) and the rest of the backup comes from an external drive. It worked fine under Mavericks, but under Yosemite it seems that Time Machine doesn't realize that the hundreds of gigabytes on the external drive has already been backed up before and doesn't all need to be backed up again.

Any thoughts? I had a backup that was several months old earlier; once I started getting this error, I wiped that backup thinking that a fresh backup might sole the error. No such luck.

I'm running Yosemite 10.10.0 on a mid-2011 MacBook Air, backing up to a Sinology NAS. (My wife who is still using Mavericks is continuing to successfully backup to the same drive.)

Thanks everyone!

- geoff