The cheapest 4TB external in stock right now is the WD My Passport 4TB Portable HDD at $31.88/TB ($128 total), a portable hdd unit. 4TB is the mainstream external capacity, where portable and desktop units overlap and the widest choice exists. It is large enough to be a real backup target for one machine and small enough to still be available in a bus-powered portable form. No maker publishes which drive is inside an external enclosure, so the table below says so rather than guessing.
25 in-stock listings · desktop, portable and SSD
Best cost per terabyte here right now: WD My Passport 4TB Portable HDD at $31.88/TB ($128 for 4TB, portable hdd, used listing). The drive inside it is not disclosed by the maker.
| Model | Capacity | Family | Interface | Inside | Warranty | Condition | $/TB | Price | |
|---|---|---|---|---|---|---|---|---|---|
| WD My Passport 4TB Portable HDD | 4TB | Portable HDD | USB 3.1 | NOT DISCLOSED | — | USED | $31.88 | $128 | Buy |
| Seagate One Touch 4TB Portable HDD | 4TB | Portable HDD | USB 3.0 | NOT DISCLOSED | — | USED | $39.75 | $159 | Buy |
| WD My Passport 4TB Portable HDD | 4TB | Portable HDD | USB 3.1 | NOT DISCLOSED | — | USED | $41.25 | $165 | Buy |
| WD Elements 4TB Portable HDD | 4TB | Portable HDD | USB 3.2 GEN 1 | NOT DISCLOSED | — | USED | $43.24 | $173 | Buy |
| Seagate Expansion 4TB Desktop HDD | 4TB | Desktop HDD | USB 3.0 | NOT DISCLOSED | — | NEW | $45.00 | $180 | Buy |
| WD My Passport 4TB Portable HDD | 4TB | Portable HDD | USB 3.1 | NOT DISCLOSED | — | USED | $45.10 | $180 | Buy |
| Seagate One Touch 4TB Portable HDD | 4TB | Portable HDD | USB 3.0 | NOT DISCLOSED | — | NEW | $45.82 | $183 | Buy |
| WD Elements 4TB Portable HDD | 4TB | Portable HDD | USB 3.2 GEN 1 | NOT DISCLOSED | — | NEW | $46.00 | $184 | Buy |
| Seagate Expansion 4TB Desktop HDD | 4TB | Desktop HDD | USB 3.0 | NOT DISCLOSED | — | NEW | $47.00 | $188 | Buy |
| Seagate Expansion 4TB Desktop HDD | 4TB | Desktop HDD | USB 3.0 | NOT DISCLOSED | — | NEW | $47.71 | $191 | Buy |
| WD My Passport 4TB Portable HDD | 4TB | Portable HDD | USB 3.1 | NOT DISCLOSED | — | NEW | $47.97 | $192 | Buy |
| LaCie Rugged 4TB Portable HDD | 4TB | Portable HDD | USB-C | NOT DISCLOSED | — | NEW | $52.50 | $210 | Buy |
| WD My Passport 4TB Portable HDD | 4TB | Portable HDD | USB 3.1 | NOT DISCLOSED | — | NEW | $59.75 | $239 | Buy |
| WD My Book 4TB Desktop HDD | 4TB | Desktop HDD | USB 3.0 | NOT DISCLOSED | — | NEW | $60.63 | $243 | Buy |
| WD_BLACK 4TB Portable HDD | 4TB | Portable HDD | USB | NOT DISCLOSED | — | NEW | $61.75 | $247 | Buy |
| WD My Passport 4TB Portable HDD | 4TB | Portable HDD | USB 3.1 | NOT DISCLOSED | — | NEW | $63.40 | $254 | Buy |
| WD My Passport 4TB Portable HDD | 4TB | Portable HDD | USB-C | NOT DISCLOSED | — | NEW | $63.75 | $255 | Buy |
| LaCie Rugged 4TB Portable HDD | 4TB | Portable HDD | Thunderbolt | NOT DISCLOSED | — | USED | $68.41 | $274 | Buy |
| WD My Book 4TB Desktop HDD | 4TB | Desktop HDD | USB 3.0 | NOT DISCLOSED | — | NEW | $81.44 | $326 | Buy |
| Samsung T Series 4TB Portable SSD | 4TB | Portable SSD | USB 3.2 GEN 2 | NOT DISCLOSED | — | USED | $132.25 | $529 | Buy |
| Crucial X Series 4TB Portable SSD | 4TB | Portable SSD | USB 3.2 GEN 2X2 | NOT DISCLOSED | — | NEW | $152.50 | $610 | Buy |
| SanDisk Extreme 4TB Portable SSD | 4TB | Portable SSD | USB4 | NOT DISCLOSED | — | USED | $161.79 | $647 | Buy |
| Samsung T Series 4TB Portable SSD | 4TB | Portable SSD | USB 3.2 GEN 2 | NOT DISCLOSED | — | USED | $162.50 | $650 | Buy |
| WD My Passport 4TB Portable SSD | 4TB | Portable SSD | USB 3.2 | NOT DISCLOSED | — | NEW | $180.00 | $720 | Buy |
| Samsung T Series 4TB Portable SSD | 4TB | Portable SSD | USB 3.2 GEN 2 | NOT DISCLOSED | — | NEW | $239.72 | $959 | Buy |
“Inside” is not disclosed by any maker for these products and the contents vary by batch, so we state that rather than guessing — see what the community has found by opening them. Warranty shows a dash because the makers' warranty pages did not resolve through our source-verification path; read the term on the maker's page for the exact model. Interface is the maker's own specification string, not a speed measurement.
New listings on both sides. Only capacities where both an external and a bare internal drive are in stock appear at all.
| Capacity | Cheapest external | Cheapest bare drive | Difference | Verdict |
|---|---|---|---|---|
| 4TB | $180 Seagate Expansion 4TB Desktop HDD | $190 Ultrastar 7K4000 4TB SATA NAS | −$10 | External is cheaper before you open it |
A cheaper external is where the case for opening one begins, not where it ends. You do not know which drive is inside before you open it, opening it ends the warranty, and some units use a 3.3V power-disable pin that stops the drive spinning up in a normal desktop. The full shucking guide covers all three.
4TB is the mainstream external capacity, where portable and desktop units overlap and the widest choice exists. It is large enough to be a real backup target for one machine and small enough to still be available in a bus-powered portable form.
At 4TB you can currently buy desktop units from $45.00/TB, bus-powered portables from $31.88/TB and portable SSDs from $132.25/TB. Those are different products rather than different prices for the same thing, and the family column separates them.
The question worth asking at this size is whether one capacity up costs meaningfully more. Cost per terabyte does not scale linearly, so the next rung is frequently cheaper per terabyte and occasionally barely more in absolute terms. For a backup target in particular, buying short is a false economy: versioned backups keep history, history grows, and a target that fills stops protecting you.
The other half of the decision is what the drive plugs into. Externals connect over USB and that is the whole interface story; there is no path to putting one in a bay without opening it, which is the subject of the shuck arithmetic above. If the storage is destined for a machine's internal bays or an array, our 4TB bare drives page covers the drives whose specifications the maker actually publishes.
The split between the two mechanical families is physical rather than commercial. Desktop externals are 3.5-inch drives with their own mains adapter; portable externals are 2.5-inch drives running on USB power alone. The 2.5-inch format tops out at considerably lower capacities than the 3.5-inch format, and that ceiling is a hardware limit rather than a marketing decision.
Both formats exist at 4TB right now, which makes this one of the capacities where you genuinely get to choose. The desktop unit is cheaper per terabyte and needs a socket; the portable costs more and does not. That difference is the entire decision, and it should be settled by where the drive will live rather than by the price gap.
A practical note that catches people out: desktop externals are not designed to be moved while running, and the mechanical failure mode for a spinning drive that gets knocked is abrupt. If the drive is going to travel at all, buy the portable and accept the higher rate per terabyte, because the alternative is a cheap drive and a bad afternoon.
The shuck table above compares the cheapest 4TB external against the cheapest bare internal SATA drive at the same capacity, on new listings, right now. That is the only version of the comparison that means anything: the general claim that externals are cheaper than bare drives is true at some capacities and false at others, and it changes week to week with stock.
Where the external is cheaper, the saving is the reason people do this. Where the bare drive is cheaper, opening an external costs money and voids a warranty in exchange for nothing at all, and the correct response to that row is to buy the bare drive.
Even a real saving comes with three costs. You do not know which drive is inside before opening it and the contents vary by batch. Opening ends the warranty on the external. And some units use a 3.3V power-disable pin that stops the drive spinning up on a normal desktop power connector, which is fixable with tape or an adapter but is an unwelcome discovery mid-build. Our shucking guide covers all three properly.
At this capacity the fake-capacity risk is lower than at the bulk sizes, because the fraud economics work best where the advertised number is large. It is not zero, and the same brand rule applies: a capacity from a name with no history outside the listing is the signal to walk away.
The WD My Passport 4TB Portable HDD at $31.88 per terabyte, $127.50 in total, from a used listing. That is computed when this page loads from our tracking of in-stock Amazon US listings and it changes — out-of-stock drives are excluded entirely rather than shown at a last-known price.
Yes — there is a bus-powered portable at 4TB in stock right now, alongside desktop units. Portables are 2.5-inch drives running on USB power alone, so they cost more per terabyte than a mains-powered desktop unit of the same size.
Check the shuck table above rather than assuming. It compares the cheapest 4TB external against the cheapest bare internal SATA drive at the same capacity on new listings today. Where the bare drive is cheaper, shucking costs you money and voids a warranty for nothing. Where the external is cheaper, you are still accepting that the drive inside is undisclosed, that opening voids the warranty, and that some units use a 3.3V pin that blocks spin-up.
No maker publishes it, and the contents vary between production batches of the same model number. That is why every row above reads "not disclosed". Our shucking guide carries what the community reports finding, framed as batch-varying observations rather than specifications, which is what they are.
Treat it with suspicion and verify it. Fake-capacity drives report a size they do not have and only fail once you write past the real capacity, which is usually after the return window. Our filters reject anything under $8/TB and any unfamiliar brand above 2TB, but if you buy outside our tables, write a full drive of real data and read it back.
The rule worth applying is that a backup target should be comfortably larger than the data it protects, because versioned backup systems keep history and history grows. Time Machine, File History and their equivalents all behave badly once the target fills. If 4TB is roughly the size of what you are protecting, buy a rung up — it is usually cheaper per terabyte anyway. Our backup guidance covers sizing and the rotation discipline.
Specification claims on this page were checked against the pages below on 4 August 2026. Where a widely-repeated figure did not survive that check, we report what the source says now and say so in the text rather than repeating the familiar number.