I have a HDD that I want to grab a lot of data off quick and got this error when mounting the drive. Tried another drive and it works fine any ideas ? 

Latest beta version out, will give it a try later. https://fatxplorer.eaton-works.com/3-0-beta/

Just passing this information on: I've just got word that FatXplorer 3.0 beta 1 has just been released for testing!
https://fatxplorer.eaton-works.com/3-0-beta/
This contains the long-awaited support of OGXbox disks!
This means no more being restricted to FTP transfer speeds to fill your disks- you can drag and drop using the ordinary Windows interface. Some features are not yet supported such as: formatting. This means you would still need to format your disk first via your XBox using XBPartitioner, then connect it to your laptop/PC.
I have noted that it does appear to support opening raw backup images, although NOT compressed ones (notably the XBMCOrigins torrent, as that comes in a custom .imgc compressed format due to being backed up by HDDRawCopyTool.). Also note that the final version will be pay-for software, although in my opinion well worth it. I've been using the 2.5 version for years for managing my 360 disks and love it.