USB Hard Drive Not Recognised

My external USB hard drive has been working fine until today. It started off the day ok and performed the scheduled time machine back up but then later in the day the Macbook Pro stopped recognising the disk. I have tried to mount the disk a number of ways and nothing seems to work. I have tried to locate it in disk utility but when plugged in this will find neither the external or internal drive. Once unplugged though it will locate the internal disk. This seems to be an issue in itself.

I have been into terminal and tried diskutil repair and I get the error

Error: -69845: File system verify or repair failed

Underlying error: 8: POSIX reports: Exec format error

So no luck in getting this to work and now I have no idea what this error code means.

I am running the beta of OSX El Capitan and I have noticed that there was an update today, could this be causing me any issues?

Any help much appreciated as I am all out of ideas and I really don't want to lose whats on my USB drive.