re: iMac 5K
Internal drives on Macs were hard drives for decades. Then we saw the transition to SSDs (solid state drives). Hard drives are much less expensive per gigabyte of storage, but make noise and are relatively slow. They are excellent choices for high capacity data storage of large files (photos, videos, etc), but most people have relatively modest amounts of data to store, so an SSD may be more appropriate.
We have now transitioned into an era where SSDs are reasonably priced but also relatively low capacity. Thus a financial incentive to buy the Mac with a 256GB or 512GB SSD. For many, that will suffice, but it is pretty easy to overflow an SSD of that size.
What to do if you need more storage and don’t want the noise and bulk of a hard drive? For the vast majority of users, it’s as simple as adding a 1TB or 2TB SSD. Plug it into the USB port, and you are good to go*.
USB-C SSDs are lightning-fast compared to a hard drive and suitable for most any use. Thunderbolt SSDs are faster still, but it won’t matter at all for most uses and users.
For most users, the OWC Envoy Pro Elektron is an outstanding choice—tiny and suitable for everyday use at home/office or for travel. At only $269 for 2TB, it hugely expands storage space on Macs with 256/512GB/1TB SSDs at very modest cost. You might even consider a 2nd one for a backup drive.
Various recommendations below. See external storage wishlist for more options.
* Depending on the vendor, the drive should be erased into macOS file system format.
$149 SAVE $11 = 6.0% OWC 1.0TB OWC Envoy Pro Elektron USB-C Portable NVMe SSD in All Other Categories
$229 SAVE $21 = 8.0% OWC 2.0TB OWC Envoy Pro Elektron USB-C Portable NVMe SSD in All Other Categories
$379 SAVE $91 = 19.0% OWC 2.0TB OWC Envoy Pro SX Thunderbolt Portable NVMe SSD in All Other Categories