---
name: "umass"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "openbsd"]
url: "https://new.carta.tech/umass"
---

# umass(4)

USB Mass Storage Devices driver

## NAME

**umass** – USB Mass Storage Devices driver

## SYNOPSIS

**device da** **device scbus** **device pass** **device usb** **device umass**

In [loader.conf(5)](/loader.conf/5.md): **umass\_load**

## DESCRIPTION

The **umass** driver provides support for Mass Storage devices that attach to the USB interface.

If the appropriate hardware is detected, the driver will be loaded automatically by [devmatch(8)](/devmatch/8.md). To load the driver manually at boot time, use the **umass\_load** command at the [loader(8)](/loader/8.md) prompt, or add it to [loader.conf(5)](/loader.conf/5.md).

To use the driver in a custom kernel, [usb(4)](/usb/4.md) and at least one of [uhci(4)](/uhci/4.md), [ohci(4)](/ohci/4.md), [ehci(4)](/ehci/4.md), or [xhci(4)](/xhci/4.md) must be configured in the kernel. Additionally, since **umass** uses the SCSI subsystem and sometimes acts as a SCSI device, it requires [da(4)](/da/4.md) and [scbus(4)](/scbus/4.md) to be included in the kernel.

## HARDWARE

The **umass** driver supports USB Mass Storage devices such as:

- USB thumb drives

- USB hard disk drives

- USB floppy drives

The **umass** driver tries its best to avoid issues with the drives, not all issues can be handled automatically, so quirks may be necessary. See the *USB* *Mass* *Storage* *quirks* section of [usb\_quirk(4)](/usb_quirk/4.md) for quirks for the drives. The **add\_dev\_quirk\_vplh** and **add\_quirk** commands of [usbconfig(8)](/usbconfig/8.md) can manage these dynamically. Quirks can be specified via tuables, as described in [usb\_quirk(4)](/usb_quirk/4.md).

## EXAMPLES

Rescan all slots on a multi-slot flash reader, where the slots map to separate LUNs on a single SCSI ID:

```
camcontrol rescan 0:0:0
camcontrol rescan 0:0:1
camcontrol rescan 0:0:2
camcontrol rescan 0:0:3
```

Typically only the first slot will be enabled at boot time. This assumes that the flash reader is the first SCSI bus in the system and has 4 slots.

## SEE ALSO

[cfumass(4)](/cfumass/4.md), [ehci(4)](/ehci/4.md), [ohci(4)](/ohci/4.md), [uhci(4)](/uhci/4.md), [usb(4)](/usb/4.md), [usb\_quirk(4)](/usb_quirk/4.md), [xhci(4)](/xhci/4.md), [camcontrol(8)](/camcontrol/8.md), [usbconfig(8)](/usbconfig/8.md).

## HISTORY

The **umass** driver appeared in FreeBSD 4.3.

## AUTHORS

The **umass** driver was written by MAEKAWA Masahide ⟨*bishop@rr.iij4u.or.jp*⟩ and Nick Hibma ⟨*n\_hibma@FreeBSD.org*⟩.

This manual page was written by Nick Hibma ⟨*n\_hibma@FreeBSD.org*⟩.