I've updated to Sys X Yosemite (10.10.5), and Time Machine now won't erase oldest back-up to make room for new one!
I have to manually erase & re-format the ext b/u disk every coupla days, then re-choose it as it has 'lost its identity'.

I keep getting a dialogue box saying "Back-up failed. Insufficient room on disk for back-up."
