Oops. I should have read your original post more carefully, and remembered what the Archos 705 actually is. In that case, your original method seems to be the only practicable one, as Archos do not give details of how their OS is installed; it could be in firmware, on the hard drive but in a partition separate from the mountable drive, or on that drive (or, indeed, some combination of these). The good news is that the OS is unlikely to present the range and complexity of problems you get with Windows, and I'd guess that the mountable drive uses the FAT32 file system, which is less complex than NTFS.
When you open the device as an external drive, is it very clear where your personal data is stored? If so, I would erase everything you recognise as yours, then erase the free space; uncheck the option to erase cluster tips, in case this causes problems. You won't, I believe cause any damage just by wiping free space, but if it fails you may need to delete the erasing files manually (this is described in the FAQ) to recover any 'lost' space.
David