internal mechanical drive vs external SSD

I recently got into a debate about about what would provide the best performance for booting an operating system, from an internal mechanical disk vs an mSATA enclosure attached with USB 3.0. His belief was that because a SSD has a better read/write rate it would be better. My thoughts are that because it is booting from an external device that it might cause latency which would might negate any benefits from the increase read/write time, or cause other problems.

• The same computer would be used for both.
• pretend that both hard drives contain the exact same OS and operate the absolute best they can.
• We understand that an internal SSD would out perform either one.

Solution 1:

The answer isn't that simple. There are different interfaces an external drive can be using, USB 2.0, USB 3.0, eSATA, thunderbolt, firewire, etc..

As long as the speed of the connection is fast enough for full throughput (such as USB 3.0 at 6 gigabit/sec) the SSD should be faster than the mechanical drive because the biggest thing that slows down a hard drive while booting is all the random access. The drive has to physically move parts internally before reading the data. The SSD has nothing to move.