Resolved Time Machine stopped 'recognising' my external HDD?

I've been using an external Toshiba hard drive to back up my rMBP with Time Machine and it was working fine to begin with, well it still is, but here's my story. Originally, I'd attach the hard drive, the icon for the hard drive would turn to blue with the Time Machine logo (the MacBook recognises the hard drive as a Time Machine) and then a back up would automatically begin. But now it's suddenly changed. I attach the hard drive, the icon remains orange as a generic USB storage device and I have to manually initiate a Time Machine back up (which then runs smoothly) however the HDD seemingly still isn't recognised as a Time Machine as is shown in the screenshot below. Maybe I'm just freaking out about nothing, but I'm confused as to why it is doing this. Any thoughts?

UPDATE: Image removed as I removed it from my Imgur. This thread can now be locked, if a moderator sees this message.