Q: External hard drive partition greyed out in Disk Utility and not recognised by Finder

I tried to connect my Lacie Rugged 2TB hdd to my MacBook Pro (Retina, 15-inch, Early 2013) running El Capitan this morning, and received an error message - I didn't think much of it at the time so didn't note it down, but it said something along the lines of "OSX could not resolve *name of drive* - files can be viewed/copied but not altered/saved"..

I tried ejecting and remounting a couple of times and now there is no error message, it simply doesn't appear in Finder at all. I opened Disk Utility and the drive is there, but the partition is greyed out and Disk Utility thinks the partition is full (which isn't the case). When I try to eject either the disk or the partition through Disk Utility, nothing happens.

I tried unmounting and remounting it through Terminal, but it wouldn't mount that way either. Out of ideas and very nervous as all my work (including some recently shot footage that is not yet backed up) is on that drive.

Any ideas?