In a world of smartphones, tablets, and ultrathin laptops, it's easy to see and use the terms "hard drive" and "SSD" somewhat interchangeably. After all, virtually all common consumer electronics these rely on Solid State Drives for their on-board memory. But when it comes to external storage, your options are a little broader. Older school hard disk drives (HDDs) are still an excellent option.
So what, exactly, is the difference, and why choose one over the other?
Disk drives have moving parts, SSD's don't.
Disk-based hard drives and SSDs appear pretty similar when you're looking at a folder on your laptop. You plug one into your device, and it appears as an extra chunk of storage. But the technology underpinning them is wildly different. Disk-based hard drives have actual spinning metal disks which house your data in a series of magnetic bits. To access your data, disk drives have to literally physically "spin up"; the parts of the disks that hold your data need to be spun underneath a tiny stylus that reads them like a record stylus reads grooves into sound.
SDD's on the other hand, do not require any movement. They are able to store your information in a series of completely immobile chips which can be accessed faster and squeezed into a smaller from. Remember how your old iPod was kind of a clunky boy? That's because it had a whole-ass disk drive in there. Modern iPhones and thumb sticks and, increasingly, external hard drives, are solid state instead.
SDD's are faster, more durable, smaller -- and expensive.
By the numbers, SSDs are pretty objectively superior to their older disk-based breatheren. They are smaller, lighter, they can access and transfer your data faster, and they're less prone to failure by jolts and drops since they don't have any moving parts. But there is, of course, one huge caveat: They are way more expensive.
My 4TB hard drive cost under $100, but WD's SSD, which has a quarter of the storage space, costs nearly twice as much. To put the tradeoffs in stark perspective, the price-per-gigabyte of the SSD is roughly 20 cents, but for the HDD it is much closer to 2 cents. But with read and write speeds that can reach as high as 1000 MB/s, the SSD is 10 times zippier than my sluggish HDD which trudges along at closer to 100MB/s.
SSDs make great working memory, HDDs are better for cold storage.
The higher price-per-byte of an SSD doesn't matter if you're made of money, but even if you aren't, SSDs can be well worth the cost. When I edit my photography, dump my photos from my camera onto the SSD because its ability to transmit data at higher speeds makes all the difference when you are sifting through shots in Lightroom, which is nigh unbearable on a sluggish HDD. Likewise, SSDs are are great for any data you'll be suffering through a loading time to get. If you plan on "working from" an external drive, an SSD's speed, durability and portability bring a lot to the table.
My clunky tank of a HDD, meanwhile, is ideal for cold storage and where I offload photos from my full SSD. From its perch on my desk, its bulk and comparably slower transfer times effectively disappear when I simply leave the room as my files transfer.