Hi nortocd
(01) It will work in your USB hard disk enclosure if the enclosure supports SATA. If the enclosure supports the older style (ATA/EIDE) interface, then you'll just need to get an enclosure that supports SATA for your new hard disk. I like the Nextar external HDD (hard disk drive) enclosures.
You can find many external HDD enclosures made by various manufacturers these days. They're available for 3.5" (laptop-sized) and 5.25" (desktop PC-sized) HDDs supporting ATA/EIDE, SATA and SCSI HDDs and even Optical drives (DVD, etc.)
Newegg is a great online source for computer & high tech-related equipment. They're prices are very competitive and their customer service is excellent. You can also find customer reviews of most of the products they sell, too, which can be very helpful.
(02) If you'd like this HDD to be portable, then I recommend you use an external HDD enclosure. They are available w/ USB and w/ eSATA connections (for connecting externally to your computer(s)). I don't recommend using IEEE1394 (firewire) as USB 2.0 is faster. eSATA is faster than USB 2.0 but is not a commonly available external connection on most computers yet.
(03) If a SATA port (connection) is not available inside your computer, you can install a SATA controller (if an expansion card slot is available in your computer) and connect your new HDD to the new controller.
(04) You can find information regarding your Compaq Presario
HERE.
(05) I checked your motherboard specifications
HERE. Your computer's original equipment does not support SATA. However, if you'd still like the HDD to be installed internally, just install a SATA controller and connect the new HDD to the SATA controller.
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Thank you,
BJ @:)