"Easy" depends whether you have any HiDef recordings you feel attached to, and how comfortable you are with the idea of installing the custom firmware.
We can offer you methods however you answer the above questions, but some will be easier than others. We could probably have saved you the bother of buying a new HDR-FOX as well - there are effective tools for overcoming some disk problems.
OK, well if as I suspect your drive has gone read-only, you will need to sort that out before there is anything we can do to rescue the HiDef recordings (at which point the old HDR-FOX will probably work normally again).
Gen up on the custom firmware (see the links in my signature panel) and then install the latest (2.14 at present). You do not need to run the web stage of the installation at this point (it probably would not work), but you will need to establish a Telnet command console session to the server you just installed on the HDR. Details about Telnet are in the Trail Guide (footnote to Stage 3B) and/or Wiki HERE.
You will get a system menu with some disk repair options - see this topic HERE (click). Maintenance Mode is what you want, detailed in the Wiki HERE (click).
Once you are booted in Maintenance Mode, connect by Telnet again and choose option 1 (fix-disk). Let us know what happens.
Following you upgrade instructions, I've fallen at the first hurdle (maybe the second). I have successfully got the .hdf file onto a FAT32 USB stick. But when I plug it into the Humax box (in standby mode) and turn the box on, I am prompted to copy the files - rather than an installation starting.
So seems like the box is recognizing the USB, but not the file type?
(Tried plugging USB into slot on the back. This time the box does not even recognize the USB.)
Correction: if we can't crack this we will have to start talking about taking the drive out and hooking it up to a PC to run a file-system repair, and then put it back (the content is encrypted and the original HDR-FOX is the only thing that "knows" how to decrypt it).
Alternatives to repairing the existing file system include preparing a new hard drive to transplant in and then transfer the existing content to it (you have a drive in the new HDR-FOX...).