The Macintosh Portable operates properly without an internal hard drive.
The Macintosh Portable does not provide termination power; therefore, an
internal hard drive is not required.
The modification you're suggesting is often used in secure environments to
ensure that storage media are not transported to unsecured areas.
Note
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Because the Macintosh Portable is sensitive to ESD damage, this or any
other modification must be done by an authorized Service Provider.