Why did my external HD fail and then come back?

⚠ The weirdest thing happened to my external hard drive and Time Machine.

I have a 2012 Macbook Pro Retina, 2.7 GHz with 16 GB of memory. It's software is 10.9.2 and has 500 GB of flash storage. So far I haven't had any gimmicks, but to be safe I use Time Machine in conjunction with a Buffalo Ministation HD-PCTU2 for backup. I purchased this with the computer (about two years). A few days ago it seemingly died --Disk Utility couldn't verify or repair it, and it wasn't mounted. I couldn't access the hard drive in Finder or anywhere else. Disk Utility couldn't tell how much memry was being used. The actual hard drive sounded and looked fine, purring away whenever plugged in. Before it stopped working, I pulled the cord by mistake without properly ejecting.

Today I wanted to buy a new external hard drive but beforehand, I used Disk Utility to make sure I couldn't repair/re-mount my old one. Still didn't work. When I was researching hard drives online, I left the hard drive plugged in. Time Machine mysteriously popped up and asks me if I wanted to use HD-PCTU2 as my backup disk! It's now on my desktop, fully accessible, and my old backup are there too. Working exactly as it should.

Any ideas about what happened? Why did Time Machine suddely recognize my hard drive after not working? Why didn't Disk Utility work? Should I play it safe and buy a new external hard drive? 😕

Thanks in advance!