There are three 2017 iMac models. Four, if you count the iMac Pro 2017. There’s 21.5”, 21.5” Retina 4K, and 27” Retina 5K. And the iMac Pro 2017. All three of the iMac models here have two Thunderbolt 3 ports, and four USB-A USB 3.0 ports.
Also... you’ve told us that you want storage, and a fairly modest capacity of two or four terabytes, but not what for. Time Machine backups? Or some other use?
The USB-C ports for Thunderbolt are also USB ports on the iMac models, in addition to being Thunderbolt 3 ports.
FireWire was no longer Part of the hardware configuration starting with ~2012 Macs.
Confusingly, USB-C is probably best considered to be a mis-named connector specification. What hardware behind the connector can be USB, Thunderbolt 3, power delivery for charging, and other protocols can be present. Different devices have different capabilities. All Mac USB-C connections available can also support USB storage and device and keyboard and mouse connections, though you’ll usually end up with a dock or hub or adapter to get to the familiar USB-A devices.
Here’s what Apple states about the current iMac 5K 27” ports:
Two Thunderbolt 3 (USB-C) ports with support for:
- DisplayPort
- Thunderbolt (up to 40Gb/s)
- USB 3.1 Gen 2 (up to 10Gb/s)
- Thunderbolt 2, HDMI, DVI, and VGA supported using adapters (sold separately)
That USB-C connector can—with the appropriate cable selected—provide all of these different connectors on the current iMac 5K 27”.
Further confusingly, the cabling connected to the USB-C connector varies by which connection will be used. Thunderbolt, USB cables with USB-C connectors, and USB-C charging cables are all commonly available, and are variously not interchangeable. Which is why I label every USB-C cable purchased.
Different computer vendors have different lists of which can be permitted with a USB-C socket, to keep this confusing, too. You shouldn’t be able to damage stuff, but the wrong cable and/or a USB-C port that doesn’t support what you want to use simply won’t work.
And to add another dollop of confusion to this morass, the USB folks decided to rename USB 3.0 to USB 3.1 gen 1, when they introduced USB 3.1 gen 2. USB 3.1 gen 1 is the same as USB 3.0. That 3.1 stuff was then replaced with USB 3.2, which is USB 3.1 doubled up. Just recently available is USB 4, but there’s not all that much USB 4 hardware available yet.
As stated earlier, hard disks have performance well below that supported by USB 3.0/3.1g1/3.1g2/3.2/4. You don’t need Thunderbolt for a modest hard disk, as hard disks are just slow.
Current-generation hard disk sizes are 14 to 16 TB. Much of anything below 4 or 5 TB is getting pretty old.
Thunderbolt storage doesn’t bother with small-capacity stuff, and tends to be reserved for use with hard disk storage arrays. Single hard disks just don’t need the performance of USB 3.0/3.1g1/3.1g2/3.2/4 speeds, much less that of recent Thunderbolt.
Don’t bother looking for small Thunderbolt disks. You’ll find SSDs and such, and SSD arrays, but USB works fine for hard disks below the size of hard disk arrays.
Five to seven years is the outer limit lifetime of most storage gear. Probably all of it, at the prices you’re probably looking to pay.
If you’re planning on keeping this for a bit while also migrating to a laptop, I’d suggest looking at FreeNAS or at a commercial NAS provider such as Synology, and one that offers Time Machine support if that’s of interest here. That so that you don’t have to remember to cable the laptop to the storage. So long as the Mac is within range of the Wi-Fi, Time Machine can run.
But for what are seemingly your current modest storage requirements, a USB 3.0-ish (3.0/3.1g1/3.1g2/3.2) external hard disk of 4 to 6 TB will work just fine. Those are typically around USD$100 or so, and most feature a USB-A connector. Expect to replace that in ~five to seven years. And Time Machine will work just fine with that.
I can’t see getting particularly invested into the purchase of a hard disk drive—particularly not one more than ~five years old—as those hard disks that do get old also get flaky and tend to fail. ~USB$100 amortized over ~five years isn’t very much, after all.
