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Is this perfectly OK or should i avoid entirely?
Technically, from an operational security perspective - it's OK. Swapping a HD and rebooting is technically equivalent to booting another computer.
However from a human behavior perspective it requires scrutiny: when your offline HD comes on, you'd need the wireless to be off. Also bluetooth, IR. Also any USB that may be connected. There's a neat hack that even involves a USB mouse to inject malware. In short, it'll take discipline.