Hi,
Not that its relevant exactly, but I know if you attach an external USB hard drive to an XBox360 USB port, it works only in Read-Only Mode....you have to pony up for a special Xbox360 hard drive to get Read/Write hard drive access....
As EddieZ pointed out it could well be an OS setting that makes it Read only...being that the Hard drive is seen as a Network Device and NOT a local hard drive, Windows tends to default to lower security settings on Network Drives....Can't speak to MacOS.... Which may end up in the read-only mode of operation.
SO I would add to your (OP) question: How does one access drive security settings for a Hard drive set up using Share Port ??