Hello,

I'm looking for a enclosure for a 3.5" drive that is USB powered (no power cord) does such an item exist?

No, the USB interface cannot power a 3.5" drive.

Even with one of those "Y" cables?

No, not even with a Y cable.

Standard USB ports do not supply the 12V needed to power a standard desktop drive. A Y cable will allow for higher amperage, but still at 5 V.

There is, technically a USB-PD (power delivery specification), but I am not aware of that implemented in any desktop computers or external enclosures.

Thanks! That is very helpful.

What blood posted is correct, 3.5 mechanicall hdds use 12V, while the USB standard is 5V, so no way of powering it via usb, you will always need an external source of power.

But you can do it with 2.5 mechancial hdds, this are laptop drives, that are design to operate at 5v, so you can build your own or the highest i seen is the WD My Passport 2TB and Toshiba 2TB Canvio

Thanks. Am I wrong to assume that the drive inside the WD My Passport 2TB has to be one of these 2GB, WD 2.5" drives ? If so why are the bare drives more expensive?

The My passport is a little lower than usual, like $130 is what it retails, but in most cases is warranty, enclosure drives usually have 1 year warranty, and bare drives most of the times 2-3 years, enterprise drives have 5 years (or used to have).

Here is another example of what you are seeing, just in 3.5 hdds,

Bare drive: Seagate Desktop HDD 4 TB SATA 6Gb/s NCQ 64MB Cache 3.5-Inch Internal Bare Drive ST4000DM000 $171.99
USB3 Enclosure: Seagate Expansion 4 TB 3.0 USB Desktop External Hard Drive (STBV4000100) $159.99 (same hdd inside)

Ah, that makes sense. If the price difference is this big (~40%) I would opt for the less expensive, shorter warrantied version. I have off-site full backups of this stuff anyway.

This isn't 100% correct. you could always include a 5V to 12V switching supply in there, it's the power limit.

The most power you can get out of a "standard" USB 2.0 port is 2.5 (5 for 2 ports) watts (10W (?) on change ports) and the spin up power, even on a "green" drive, is more than that. With a changing port, you could do this, but then you'd have to say that, and few devices have that. You could add some kind of power storage to get the spin up done, but that would add up to to many issues/$$$. You might be able to do this with 2 3.0 ports (4.5W/port) but you would still have to limit the drive selections, and still require 2 ports.

If they ever come out with the new high power ports (up to 100W), then you could. (these make sure the requesting device can handle the spec, then up the 5V line to 20+ volts) (ASUS tables (others???) do this on their changer)

My suggestion is as above either go for external 2.5" drives (which are USB powered)
Or possibly consider a docking station as a solution. I can recommend the ANKER USPEED as a good solution USB 3, and eSATA.

It's worth thinking about if you are doing lots of offline storage, also the dock can be very useful for other pc work such as backing up and OS re-installations, data recovery etc etc. It's really not that big and will fit on your desk quite easily.
And yes the ANKER is powered via an adaptor.

That's how I work things anyway with the dock AND 2.5" drives

Thanks Mr. Spocko, but I think I'll go with the 2.5" device powered off of USB. This is for a home file server that is a retired laptop. When we have storms in the mountains up here we sometimes lose power for a few minutes at a time. With an externally powered drive, this requires an umount/fsck/mount cycle to get things working again. With a USB powered device it should be able to ride out the power outage on the laptop's battery.