Drive Enclosures - DAS, JBOD and What Each One Needs

DAS, JBOD and NAS are three different purchases

The words get used interchangeably by sellers and they describe genuinely different machines. Getting them confused is how somebody ends up with a box that cannot do the thing they bought it for.

A DAS - direct attached storage - is a box of drives that plugs into one computer, usually over USB or Thunderbolt. It has a controller inside it, the computer sees the drives, and there is no network involved. It is fast, it is simple, and it is attached to exactly one machine at a time. Unplug that machine and the storage goes with it.

A JBOD or expansion enclosure is dumber than that and it is the distinction that catches people. It is a shelf of drives, a backplane and a power supply, with no controller a computer can talk to. It needs a host bus adapter in the machine and an external cable between the two. Bought alone it will power on, spin the drives and be completely invisible to the operating system - this is the single most common expensive mistake in this category and it is why the warning block above sits over those rows.

A NAS is a computer. It has a processor, memory, an operating system, and it serves storage over the network to everything on it at once. It is the only one of the three that several machines can use simultaneously, and it is the only one that can run anything - backups, media indexing, containers. It also costs the most, and if you have one machine and want more space on it, you are paying for a computer you did not need.

The rough decision: one machine and one user, DAS. Several machines or several people, NAS. A server that already has PCIe slots and needs another dozen bays, JBOD.

The USB class: what you are actually buying

USB enclosures are the volume end of this market and the specification that matters is not the one on the front of the box. Every listing leads with a transfer figure - 5Gb/s, 10Gb/s, sometimes 40 - and that figure describes the bus, not the drives.

A four-bay enclosure full of mechanical drives will not saturate a 10Gb/s USB link no matter what the box says, because the drives inside it are the limit. The bus speed only becomes the constraint with SSDs, or with a large number of drives being read simultaneously. We publish no throughput figures of our own for any of this hardware - we do not benchmark it, and repeating the seller's number as though we had measured it would be worse than saying nothing.

What does matter, and what listings are much quieter about: whether the enclosure presents each drive individually to the operating system or hides them behind its own RAID controller. The first is what you want if the data matters, because it means the drives are readable in any other machine and your array is managed by software you can debug. Hardware RAID inside a $150 enclosure means that when the enclosure dies, the array is a puzzle rather than a set of disks.

The second quiet specification is whether the enclosure spins drives down and how aggressively. Enclosures that park drives after a few minutes of inactivity, and then take fifteen seconds to wake, make a backup target feel broken. It is rarely in the listing and it is worth searching for before buying.

The JBOD class, and the card you have not budgeted for

An external SAS enclosure is the cheapest way to add a lot of bays to a machine that already exists, and the sticker price is not the price. The box needs a host bus adapter with external ports, and the HBA needs a free PCIe slot in a machine that has one.

The connector generations are where the money gets wasted. SFF-8088 is the older external mini-SAS; SFF-8644 is external mini-SAS HD, the 12Gb/s generation. They are different physical connectors. Worse, the internal connectors of the same generations - SFF-8087 and SFF-8643 - look broadly similar and do not mate with the external ones at all. A card with four internal SFF-8643 ports has nothing to plug an external enclosure into, however many ports it has.

QNAP's SATA JBOD line is the same requirement wearing different wires: those units ship with a PCIe SATA host card rather than using SAS, and the enclosure is useless without it. Same rule - a free PCIe slot and a card in it - so we file both under one class on this site and keep the actual wire in the connection column rather than calling a SATA JBOD an SAS one.

The upside is real, which is why people do it anyway. Second-hand external SAS shelves are cheap relative to the number of bays they carry, an HBA in IT mode presents every drive raw to the operating system, and ZFS or mdadm on the host does the rest. It is the standard way to get to twenty-plus bays without buying a chassis that holds twenty-plus bays.

Filling one, and what the drives cost

An empty enclosure is not a storage purchase, it is a third of one. The basket below prices the box with a drive in every bay, at our current tracked prices, because the total is the number that decides whether this is the cheap path.

The drives are the same drives that go in a NAS and the same selection rules apply. For a backup target that spins occasionally, cheapest per terabyte is a defensible choice. For anything running continuously, the workload rating and the recording technology matter, and our capacity pages carry both.

This is also where shucking becomes relevant, and it is worth being blunt about the trade. External drives are frequently cheaper per terabyte than the equivalent bare drive, and the drive inside the shell is often the same platter. What you give up is the warranty and, on some models, a power pin arrangement that needs taping before it will run in a standard bay. Our shucking guide covers which models are which - the point here is only that an enclosure plus shucked drives is a genuinely different price from an enclosure plus retail drives, and neither of them is the price on the enclosure listing.

One practical note that costs people a return: bay height. Most multi-bay enclosures take 3.5-inch drives and will also take 2.5-inch ones with an adapter or a tray that supports both, but not all of them do, and a listing that says four bays does not always say four bays of what size.

What we are not telling you, and why

There are no throughput numbers on this page and there will not be. We do not benchmark enclosures. The transfer figures in the listings are bus specifications rather than measurements of the box, and reprinting them beside our own tracked prices would let them borrow a credibility they have not earned.

There are no thermal figures either. Enclosure cooling is genuinely important - a four-bay box with one small fan and drives packed against each other will run them hot, and heat is the main thing that shortens a drive's life. But we have not measured any of these enclosures and we are not going to publish a temperature we did not take.

What is on this page is what we can stand behind: the price, in stock now, from a listing we re-verify; the bay count, the interface and the host requirement, read from the maker's own description; and the arithmetic of what filling the thing costs. Everything else is a link to the vendor.

Where to go next

USB DAS · JBOD / expansion · HBAs and IT mode are the card a JBOD needs · Shucking is often the cheapest way to fill one · SATA drives and NAS drives are the two shortlists · NAS devices if the answer turns out to be a computer after all · Drive docks for one or two drives at a time · add power protection - an enclosure counts toward the load.

Frequently asked questions