Or, if you do not want to spend that much money on a X5, the Samsung T5 is great drive as well. Since the drive is USB 3.1 vs. TB3 it is no where near as fast, but I haven't notice any issues with the speed. I have my iTunes collection as well as my Steam game collection on the T5 drive and I have had zero throughput problems.
The T5 has a SATA III drive inside. USB 3.1 doesn't mean a thing here because the SATA III drive is the bottleneck. eSATA, Thunderbolt, USB 3 and 3.1 are all the same speed because the drive inside is slower than any of those protocols.
If you accidentally use a USB 2 cable (USB-C is a port and some of the cheap ones are only USB 2), that will make a T5 slower but nothing can make it faster. If a USB-C cable advertises
480 Mbps transfer speed, that's a USB 2 cable in disguise.
5 Gbps is USB 3.0;
10Gbps is 3.1 and Display Port for video.
40Gbps is Thunderbolt 3 and lesser cables will Not pass TB3.
So it's 1/6 the speed. As you have observed, that's not an issue for iTunes. Many types of files are fine with that. Archive projects onto it.
You do not want to use a T5 for hosting active DAW projects, Photoshop, FCPx or anything else requiring speed and a wide data path. If doing any of those tasks and your Mini's internal drive is too small, get an X5.
When Time=$$$, the X5 is cheap. It's also the same price as adding the same capacity when a Mini is new. Interestingly, if you can find a 1T or 2T internal storage on a Mini in the Refurb Store, an X5 is actually more expensive.
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When connected to a 10Gbps USB port as found in current Macs, the Samsung T7 Touch is almost twice the speed of the T5 and is only slightly more expensive than the T5, so I'd consider the T7 over either the X5 or T5.
The T7 has an NVMe blade in it instead of a SATA III drive which accounts for the speed.
For my needs, the X5 is the only choice since it is nearly as fast as the internal storage— but I'm not you.
With the kind of work I do, the ability to restore deleted files within a minute or a complete system restore in less than 5 min is very important. That can only be done through APFS Snapshots on the System drive if Time Machine is enabled. Near instant Snapshots restore cannot be done on any files located on external drives, even an X5.