You would need to go to Disk Utility & check the drive is formatted as not only Mac OS X Extended, but that the underlying structure is GUID Source: Macworld - Time Machine tips and troubleshootingĪpple's KB doesn't seem to quite be so explicit - OS X Yosemite: Disks you can use with Time Machine So if you have, say, a 750GB or 1TB drive, you must repartition it to use the GPT or APM scheme before it can work with Time Machine. Although OS X supports MBR, there’s a catch: Mac OS Extended volumes can be no larger than 512GB on a drive partitioned with the MBR scheme. Drives originally set up for use on Windows usually use the Master Boot Record (MBR) scheme, as opposed to the Apple Partition Map (APM) scheme, the default for PowerPC-based Macs, or the GUID Partition Table (GPT) scheme, the default for Intel Macs. The partition map scheme describes how the drive stores its volumes.
This article is quite old, but I think is probably the reason…
