What is the brand and model of your Freeview box?
If the receiver is Linux based, it is quite possible the box has formatted the HDD in a file system incompatible with Windows, such as Ext2/3. A simple test would be to get and boot a Live Linux disc and see if the HDD mounts.
Another good possibility is that it uses a proprietary system designed to prevent users from accessing content, like Sky’s set-top box HDDs.
As a test to prevent this issue happening again, try connecting a blank FAT32-formatted USB flash drive and see if it can record to it without the box requesting to format it. My set-top box stores recordings as TS files, which can be played back in VLC or converted to MP4 with HandBrake.