This page says:
And, not all USB disks can be presented to a VM as a passthrough disk.
That is the important part.
If the driver is strictly user mode the USB disk cannot be presented. Only a select few devices presente themselves in a way that the system (outside the user session) can properly access and control the disk.
Suppose I wanted to use this functionality, but then needed a replacement for my USB disk. How would I tell before buying if it has this capability?
Note: I am assuming, from the context, that this is separate from the capability to "offline" the disk, which, AFAICT from posts elsewhere, is (thought to be) 1:1 tied to whether it's a spinning USB hard disk rather than a stick or other flash/SSD-ish beast. If this isn't correct, apparently I'd also need to know before buying whether a particular USB disk is offline-able.
There's no universal way to tell this before buying a drive as it is not a specification that anyone ever lists explicitly. However, the general rule, and reasoning is as follows:
All standard USB drives will be standard block devices. Any USB mass-storage (block) device should work for passthrough. The only exceptions are where USB drives have proprietary overlays or segmentation features. A non-exhaustive list of examples where this occurs is below. I'd posit >99% of non-encrypted drives will just work fine, the user-mode device presentation concern is a very, very unlikely issue to actually occur.
Some drives that uses a portable 'encryption' applications will be user-mode. In this case, an application loads a file or partition on the drive, decrypts it on-the-fly, and then presents a virtual volume to the OS. In other encrypted drives, the user mode application simply supplies an unlock/decryption key to the device, at which point the device hardware itself performs decryption and presents as a standard, unencrypted block device. In the second case, passthrough may work but may be unreliable, as the actual storage device doesn't usually show up until certain software is run that supplies the correct unlock code. Also, some drives will disconnect, and reconnect as a different device when unlocking is triggered.
Some drives that are larger than certain limits - e.g. LBA-32, MBR, SD(non-HC) etc. - most common in >2TB drives, will present as multiple, virtual 2TB drives. How they do this again varies as above, but can involve a software component. Devices that have a hardware switch to switch between two partitions tend to be OK.
Finally, some drives with funny features (particularly common in older gimmick drives and WiFi sticks or 3G modems), will present as multiple separate USB devices, sometimes connected to a hub. For example, some USB drives will present as a read-only CDROM, and a second USB block device. Some will only present as a USB CDROM with drivers on it, and once those drivers are installed, the CDROM disappears and a block device appears (or modem, etc.). Again, these are likely to work inconsistently with pass-through.
As a note, most drives that come with "one touch" backup software or similar, will be normal block drives, with the optional software just as a bunch of files in a folder in a normal filesystem - these can be used just fine, just ignore the extra software.
As a second note, from my earlier comment: the capability to "offline" the disk, has nothing to do with whether it's a spinning USB hard disk rather than a stick or other flash/SSD-ish beast
The only thing it's 1:1 tied to is what sort of device the OS and controller driver presents as. In other words, a controller may present disks as removeable or not, but all disks of any type connected to that controller will all show as removeable regardless of what kind of disk it is. Certain SATA drivers will do this, others will not, same for USB.
AFAIK though, the ability to 'offline' a disk isn't actually necessary.