The HB-1000S will not record to a memory stick only a USB hard-drive (unpowered recommended).
A hard-drive can be attached in a number of ways – one as a dedicated device (recording, playback, buffer functions) or as a USB device for playback. I have not verified the playback only mode except with a memory stick. In the first mode I imagine, from the warning message, it is rendered unusable (without reformatting) by other devices.
The HB-1000S can also play back a range of video formats as a DLNA client – this may be the way to go if your “Seagate portable storage device” is a DLNA server.