MBP + External HD acting strange

I just had the logic board replaced on my '08 MBP after being bitten by the faulty Nvidia chip and upon connecting my external hard drive, things got strange. The amount of purportedly used space on my external grew by some 80 GB out of nowhere, while the Time Machine backup file supposedly decreased from 90 GB down to 22. At the same time, Time Machine won't read the backup and claims to need 105 GB to create a new backup (when I'm only trying to back up 87 GB worth of data from the primary drive to begin with). So essentially, even if I delete the months worth of backups that are/were once contained on the external drive, I still don't have enough space to create a new one due to the sudden appearance of all this new unaccounted for used space. Has anyone else run into anything like this?

After making this post, I went though all of the folders on the external drive and checked them for usage one by one (both the hidden and unhidden folders) and came to a total of just around 200 GB. But every indication is that 415 GB is currently being used. This is nothing new either...it's shown upwards of 340 GB used for several months now. That's an awful lot of unexplained disk usage.