Hi All. I'll try to be concise. 13" Macbook Pro early 2011. OS = 10.9.5. Trying to backup 1TB internal hardrive on 1TB external hardrive dedicated exclusively to Time Machine backups. Never had any trouble until recently. On my internal, I'm only using 493 GB of 1TB. When I try to back up, the "Preparing Backup" takes forever and then it eventually says it failed. See photo below. I've deleted the "inprogress" file and there is currently only 1 backup on the external HD. How are only 348Gb available on my external and why would it require 458Gb to successfully backup? Is this due to older files that may have been stored on my Mac at one time but no longer there? [At one point, the internal HD was almost full with media that's since been transferred to a stacked external HD for work]. Do I need to have an external HD that's larger than my internal HD to appropriately back up my computer? Happy to provide more details if that helps. Thanks for any advice.