No longer able to backup with Time Machine
I have backed up my 2.93 GHz 640 GB iMac with 4 GB of memory for years to a 1TB external drive via Time Machine. I'm using OS X version 10.7.5. I've started to run out of space on the iMac so I got a 2TB external drive to move my photos to. Before I did anything, I made sure I had multiple backups on both Time Machine and my cloud storage (Back Blaze). I had backups going back over a year. I moved my photos via Lightroom and have been able to create one successful backup but now I get an error message that there's not enough space on the 1TB to backup and only the one backup exists in Time Machine. I've run a Disk Utility on the external backup drive and there are no issues. I'm not understanding how all of a sudden I can no longer back up when I have the same amount of data, actually less since I deleted some duplicate files. And all the old backups have disappeared. I'm concerned that I only have one backup from last night. It's as though that one backup is huge. When I try to click on the folder to check the folder size, the drive keeps churning and never returns the file size. Is this a sign of a corrupted or damaged drive? Any other options or troubleshooting?
iMac, Mac OS X (10.7.4)