If you just installed the disk from the HDR-FOX in the YouView box and switched it on, it probably did nothing to it unless you went into maintenance mode and did a factory reset or internet/ USB recovery. Did the YouView box progress past the 'please wait' screen when you switched it on? If it did you are out of luck: the YouView formatted disk has seven partitions in XFS format (some encrypted) and your old data will have been overwritten.
If your disk has not been formatted by the YouView box, your best bet is to reinstall it in the HDR-FOX. If you have a HDMI green screen problem, try connecting to the TV with a scart lead. The picture will not be as good but if your programmes are still there you will be able to watch them. If you then want to copy them and watch on a PC, for example, have a look at Black Hole's guides and the wiki as the recordings will need to be decrypted.
Even if your recordings have survived you will not be able to watch them on the YouView box. There is no playback via USB on YouView and while it is possible to copy them to a YouView formatted disk, they will be deleted when the unit boots up.