No, it doesn't have room for a 3.5" hard disk.
It is, however,
designed to accept
two 2.5" hard disks.
There are even instructions in the FAQ section of this BBS on how to upgrade the hard disks yourself. At the current time, the biggest user upgrade was to install a pair of 48's in the unit for a total of 96 gigabytes of storage in the dash of your car. Mind boggling, isn't it?
Just out of curiosity, why is a 3.5" hard disk important? 2.5" hard disks are getting cheaper, and since they're designed for laptops, they are more rugged and better suited for mobile use.
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Tony Fabris