Welcome to the forum.
Unfortunately, you have been instructed incorrectly.
An internal will always be the best. Having a minimum of 2 physical internals is where to start and having 3 physical internals is better, and worth the $, so long as you have the space and controller connectors.
With externals, they stack up this way:
1.) USB 2.0 - horribly slow and prone to read/write errors, when attempting to edit to/from. Should be used for archiving and backups.
2.) FW-400 - slow, but better. Not prone to read/write errors, like USB 2.0.
3.) FW-800 - faster yet. I use these to edit to/from, and can migrate my Projects from computer to computer
4.) eSATA - now we're talking. These will offer real world performance, just a bit below your internal SATA II HDD's
Do not have data on USB 3.0's, or SATA III's. SCSI is good, but require a dedicated controller, and are not that large. They also cost premium $'s, and the performance is not THAT much better than a well-turned SATA II.
SSD's are good for OS, but one should have 2x physical HDD's for actually editing. The $/GB ratio is still way up there.
Things to avoid:
1.) partitions on any HDD
2.) only one RAID 0, which will be seen as a single HDD. RAID's are great and fast, but having multiple physical HDD's is better for video editing. If one uses RAID, it would be best for the media HDD and for the Export HDD. Some also use them for a separate Audio HDD.
PS - all will benefit from regular defragmenting. I do this daily, if not more frequently.