I've been regularly backing up my 2011 MacBook Pro using Time Machine with two separate external drives (1Tb at work, 2Tb at home) while running Mavericks.
Today I upgraded to Yosemite, and Time Machine seems to think I have a brand new computer - the backup on the 1Tb disk failed with "not enough space", and the backup at home is currently attempting to back up the entire 350Gb on my computer.
(Previously with the 1Tb drive, Time Machine would automatically delete old archives when the drive was full.)
Is this a common problem (ie a bug)? Is there a way to tell Time Machine to continue where the last backup finished, rather than starting a brand new "branch"?