Upgrading my storage. Best method? Internal External?

You say, "can only handle 5 HDDs". But have you considered the option of getting a small adapter kit that allows you to mount a standard 3½" hard drive in an unused 5¼" slot? Many machines have such slots with front access panels removable, but are not using them for any device. Leave the front cover in place, adapt the smaller drive to the space, and find power supply connectors (use a 2-from-1 adapter if necessary) and an unused SATA mobo port to connect.

But to your questions. First, can you put the SATA II drive in your existing WD MyBook in a different case with an eSATA connection to your computer? Yes. I assume you have (or can make) a proper eSATA connector or port. Just be sure the case you buy has an eSATA interface system to the computer (many come with more than one interface included) and an INTERNAL interface for a SATA II drive.

Adapter to mount the IDE drive in the MyBook case? I doubt it. Usually there's not enough extra space inside to allow for fitting the adapter between the SATA II connector and the IDE drive.

I'm not a fan of RAID0, so I won't comment other than to say you really should check out real-world recently-measured disk performance data relevant to this. I suspect you'll find that the blacks alone are VERY nearly as fast as a RIAD0 constructed from them for most applications. Balance that against the increased risk of total data loss with RAID0 arrays.

Thank you very much for your response Paperdoc.

Does anyone else have any input?

Thanks!