What is the fastest and efficient external storage device to use with a MacBook Pro?

An external SSD with Thunderbolt 3/4 connectivity is the fastest.

Irishmonk wrote:

What is the fastest and efficient external storage device to use with a MacBook Pro?

You do not specify what exact Mac you are using— not all MacBook Pro's are the same...over all the years...(?)

HD or SSD in your exact Mac?

Many connections are backwards compatible yes, however r/w copy speed is going to limited by the 'weakest' link in the chain.

Don't waste your money on Thunderbolt if you do not have that port for example.

Not only a hardware question but also a formatting is part of the equation...

please advise

I don't know of any external storage device that advertices "saves time and money". If it were me getting a new external storage device it would be an SSD.

Please define "efficient external storage device".