Does anyone know of any software that can emulate a USB flash drive through an available USB port in OS X? Perhaps some way to map a directory to a USB port that could then be connected to another device that supports reading USB storage devices?

I'd love to connect my laptop to my car's USB port and access files as if it were a USB drive. I know about the target disk mode with firewire (not sure if this is also supported over USB), but I was hoping for something that doesn't require booting outside of the OS (I want to retain use of the machine).

I'm thinking there may be hardware limitations that prevent software from doing this by itself.

Any ideas?

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    Can you say more about what you're trying to accomplish? You want the car to access your laptop's files? Laptop to access the car's(?) files? What's on the other side of your car's USB port? – JRobert May 21 '10 at 18:59
  • I have similar questions as @JRobert. I'm curious, are you talking about your car stereo, or your car's satnav, or your car's engine computer, or what? – Spiff May 21 '10 at 19:29
  • To clarify, I want to take advantage of my stereo's support to play audio from a USB storage device. – Wilco Sep 12 '10 at 19:22
  • It wouldn’t work also for the reason that your car needs exclusive access to the USB drive. Remember that firewire target disk does not work when an OS is started. – kinokijuf Jan 2 '14 at 11:03

Unfortunately, USB makes a firm distinction between the Host and the Device. If the USB receptacle on your car has the standard USB A (Host) connector like a PC would have, it means the car's computer wants to be the Host, just like your computer wants to be the Host, so they would most likely conflict with each other and not be able to talk to each other.

Some people have made "USB file transfer cables" for connecting two Hosts together. These cables have an embedded chip that does the work of making the two Hosts appear as devices to each other. I believe these products assume both Hosts are full-fledged PCs, not embedded systems that happen to have a Host connector. I don't have experience with these cables, so I'm not sure whether a software install would be required on one or both ends. It might be worth looking into.

The USB Implementers Forum (the body behind the USB spec) eventually realized the limitations of the Host vs. Device distinction, and created the USB On-The-Go (OTG) specification as a way for USB-capable things to switch between the Host and Device roles on the fly as needed, depending on what they're talking to. However, I think you'd probably need both ends to support USB OTG.

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    A lot of the A-A cables are actually just dumb cables. Normally made to enable connection of some dumb manufactures hardware that uses the wrong socket. If you are looking at buying one that stands any chance of working make sure you are careful about what you buy. – pipTheGeek May 21 '10 at 19:50

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