What SSD should you buy? In normal times, we'd simply tell you to follow our Best Storage guide. But during flash sales, like in Black Friday or the holidays, recommendations that are based on normal prices become somewhat irrelevant since the best deals often go to lesser known drives (models, capacities, etc.), which may be worth buying at a healthy discount.

In that scenario, you may be faced with several questions such as... is QLC as good as TLC? Do SSDs really need DRAM? Why do SSDs have different shapes? Does the SSD's capacity affect its performance? This short guide will guide you to learn the basic differences between all types of consumer SSDs, so when you see an SSD on a sale, you'll know whether it's a good buy for you.

NVMe vs. SATA SSD

The interface of the SSD determines not only transfer speeds, but whether you'll be able to install it in your system. For years, SSDs used the same SATA interface as hard drives, and were either similar in form/shape to 2.5" drives that were used in laptops; or used the more compact mSATA form factor, which was similar to Mini PCIe used by devices such as network cards.

With SATA 3.0 becoming a limitation to transfer speeds at about 560MB/s, the NVMe interface is effectively replacing it, connecting to the CPU directly, or through the motherboard's chipset with several PCIe lanes, for much faster speeds.

Many motherboards have more connectors than they can utilize at the same time, so regardless of your choice of SATA or PCIe, you should check whether using a connector in that mode would disable another one that you need.

The Crucial MX500 is about as good as a SATA drive can be. If you want an NVMe drive, the Western Digital Black SN_750 currently offers great value.

Add-In Card vs. M.2 SSD

Most SSDs today, SATA and NVMe, use the M.2 form factor, which supports up to four PCIe lanes for an NVMe SSD.

With M.2, PCIe 3.0 SSDs enable transfer speeds of up to 3,500 MB/s, while PCIe 4.0 SSDs enjoy speeds of up to 7,000 MB/s, as long as your CPU and motherboard support the faster 4th-gen interface.

Most M.2 SSDs are notched according to the M key, which supports up to four PCIe lanes and SATA. Some older motherboards have M.2 slots that support the B key and only two PCIe lanes in addition to SATA.

Most SSDs that use SATA or two PCIe lanes are double-notched according to both keys for compatibility, though...

All M.2 SSDs are 22mm wide. The most common ones are 80mm long, and called "2280." Laptops, and mostly ultrabooks, sometimes only have the space for 42mm-long SSDs, called "2242." Tablets such as the Surface Pro 8 use 30mm-long SSDs ("2230"). SSDs that are 60mm long ("2260") are widely supported, but not common. The few that are 110mm long ("22110") are not supported by mainstream devices.

As an alternative to M.2, some PCIe SSDs come in the form of add-in cards, looking like small graphics cards and installed similarly. The larger form factor can make up for a motherboard's lack of PCIe 4.0 support by using eight PCIe 3.0 lanes, or accommodate a more powerful controller that needs better cooling. Another alternative is a 2.5" U.2 SSD, which can be connected to an M.2 slot with an adapter cable.

Western Digital's AN1500 is probably the fastest SSD when attached to a PCIe 3.0-only motherboard or CPU. If your system supports PCIe 4.0, then Samsung's 980 Pro is a top choice.

QLC vs. TLC SSD

In modern SSDs, the cells of the flash chips are made of levels, with each level storing a bit (0 or 1) of data. Most SSDs today use either tri-level cells (TLC) or quad-level cells (QLC). The term "multi-level cells" (MLC) was originally used to describe dual-level cells, but the term "3-level MLC" used by Samsung simply means TLC.

Adding levels to cells allows them to store exponentially more data in the same physical space, but also makes them exponentially slower to write to. The good news is, you won't notice that immediately thanks to smart caching mechanisms.

Most SSDs use a portion of their free storage space as a cache of virtual single-level cells (SLC) by writing only to the first level of the cells. Once the cache depletes, the drive degrades to its "native" writing speed. In the case of QLC, that speed might be similar to that of a hard drive.

Whether it has QLC or TLC, the less free space your SSD has, the smaller its SLC cache will be, and the shorter the time for which it will be able to sustain its top writing speed.

If you really need an 8TB SSD, then the Sabrent Rocket Q is the best choice for you. If you can get by with 4TB or less, the company's Rocket 4 Plus will perform more consistently thanks to its TLC flash.

DRAM-Less vs. DRAM-Equipped SSD

In order to map where data for each file is physically stored within flash chips, most SSDs rely on their own local RAM – typically 1MB of RAM for every GB of storage space – but that isn't always the case.

NVMe SSDs often use the host-memory buffer (HMB) to utilize some of the system's RAM for the task. In shorter M.2 SSDs, that may be done in order to save physical space. In larger SSDs, the purpose is to save costs.

When a drive that uses HMB is almost empty, the lack of on-board DRAM won't hurt its performance noticeably. If you store hundreds of GBs of data on it, however, the speed at which it finds files can become several times slower (but still many times faster than a hard drive).

With SATA SSDs, things are more complex. Instead of the main system's RAM, DRAM-less SATA SSDs use their own flash chips, which are much slower than any kind of RAM. In addition, storing the ever-changing index of all of your data on the flash chips can make them wear out more quickly and hurt the device's life span. For that reason, we can only recommend a DRAM-less SATA SSD as a temporary solution.

If you are looking for a short NVMe drive, then Sabrent's DRAM-less Rocket Nano is your best bet. Instead of buying a DRAM-less SATA drive, you should look for something like the Western Digital Blue SSD (2018), which is never far behind the best SATA drives, and often cheaper on sales.

250GB vs. 500GB SSD

In the past year, the demand for low-cost PCs for working from home has made 250GB SSDs almost as expensive as their 500GB versions. During a flash sale, however, a 250GB SSD can suddenly cost the same per GB as a similar drive with double the capacity. The question is, will the 250GB drive be a good value in that situation?

That may not be the case for two reasons: 1) even if they use the same percentage of their free space as SLC cache as higher-capacity drives, smaller drives still have smaller SLC caches to begin with. 2) Because they use less flash chips, they may not take full advantage of a controller that was designed to write to several chips simultaneously.

In NVMe drives, you may notice it immediately: for example, the Samsung 980's (non-Pro) 250GB version is rated for a top writing speed of 1,300MB/s, while the 500GB version is rated for double that speed.

In SATA drives, you may only see the difference after the SLC cache fills up. The Crucial MX500 250GB and 500GB versions both start long writes at about 450MB/s, but when their SLC caches are full, the 250GB version drops to 200MB/s, while the 500GB one stays at a respectable 400MB/s.

If you want a good 500GB SSD for an affordable price, consider the Samsung 980 (non-Pro). Instead of buying the 250GB version, you should look into something like the 500GB version of Western Digital's Blue SN550 for a similar price.