osx - Emulate a USB port as a USB flash drive?

07
2014-07
  • Wilco

    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?

    • Answers
  • Spiff

    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.


    • Related Question

    linux - Partition table corrupted (USB flash drive)
  • Questioner

    It's an 8 GB Patriot thumb drive, which I've used extensively with lots of data. Today, it is detected, but all data is gone: (EDIT at least some data is still there, but the partition table is gone)

    EDIT @Sathya (thanks) here's the relevant output from sudo fdisk -l:

    Disk /dev/sdc: 8019 MB, 8019509248 bytes
247 heads, 62 sectors/track, 1022 cylinders
Units = cylinders of 15314 * 512 = 7840768 bytes

Disk /dev/sdc doesn't contain a valid partition table

    It looks like it is /dev/sdc, with that 8 GB... and no partition table.

    I tried to mount /dev/sdc (and then dmesg | tail):

    /media> sudo mount /dev/sdc mytmp
mount: wrong fs type, bad option, bad superblock on /dev/sdc,
       missing codepage or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

/media> dmesg | tail
[   24.300000]  sdc: unknown partition table
[   24.320000] sd 2:0:0:0: Attached scsi removable disk sdc
[   24.370000] usb-storage: device scan complete
[   26.870000] EXT2-fs error (device sdc): ext2_check_descriptors: Block bitmap for group 1 not in group (block 0)!
[   26.870000] EXT2-fs: group descriptors corrupted!
[   50.420000] unhashed dentry being revalidated: .DCOPserver_eeepc-brendanma__0
[   50.430000] unhashed dentry being revalidated: .DCOPserver_eeepc-brendanma__0
[   50.430000] unhashed dentry being revalidated: .DCOPserver_eeepc-brendanma__0
[ 5565.470000] EXT2-fs error (device sdc): ext2_check_descriptors: Block bitmap for group 1 not in group (block 0)!
[ 5565.470000] EXT2-fs: group descriptors corrupted!

    EDIT @Col: results from testdisk

    Disk /dev/sdc - 8013 MB / 7642 MiB - CHS 1022 247 62
Current partition structure:
     Partition                  Start        End    Size in sectors

Partition sector doesn't have the endmark 0xAA55

    After I hit [proceed], it says:

    Structure: Ok.


Keys A: add partition, L: load backup, Enter: to continue

    The "Structure: Ok." seems reassuring... will "A: add partition" make my old data accessible (if it's still there), or will it make a new, fresh partition?

    Another option is "[ MBR Code ] Write TestDisk MBR code to first sector" - would it be better to do this?

    EDIT I found that at least some of my data is still on the flash drive, by using the below, and searching for English text in less (like " the "):

    cat /dev/sde | tr -cd '\11\12\40\1540-\176' | less

    (The drive changed from "/dev/sdb" to "/dev/sde" because I connected some extra drives today). I've learnt that "/dev/sde1" would be the first partition; and "/dev/sde" is the whole drive. Because unix treats these devices just like files, you can use all the ordinary unix file commands on them, like cat, and then process them like any other stream of data. The tr above removes non-printable characters ("\40" is space, which I wanted to preserve). In less, you can use "/" to search, similar to Vim.

    How can I get my data back (assuming it's still there)? If only the partition table is corrupted, is there a standard "partition recovery tool"? Is there a way to "repartition" without deleting everything?


    • Related Answers
  • Col

    It may be worth running testdisk over the device, it sounds like your partition table may have gone pop. Testdisk should be able to recover your data.

  • Seasoned Advice (cooking)

    ddrescue is the tool you need. I don't think you'll be able to salvage the partition table and access the data any other way, based on the output you've posted from testdisk etc.

    http://www.forensicswiki.org/wiki/Ddrescue

    ddrescue is a data recovery tool. It copies data from one file or block device (hard disc, cdrom, etc) to another, trying hard to rescue data in case of read errors.

    The basic operation of ddrescue is fully automatic. That is, you don't have to wait for an error, stop the program, read the log, run it in reverse mode, etc.

    If you use the logfile feature of ddrescue, the data is rescued very efficiently, (only the needed blocks are read). Also you can interrupt the rescue at any time and resume it later at the same point.

    Ddrescue does not write zeros to the output when it finds bad sectors in the input, and does not truncate the output file if not asked to. So, every time you run it on the same output file, it tries to fill in the gaps without wiping out the data already rescued.

    If you have two or more damaged copies of a file, cdrom, etc, and run ddrescue on all of them, one at a time, with the same output file, you will probably obtain a complete and error-free file. This is because the probability of having damaged areas at the same places on different input files is very low. Using the logfile, only the needed blocks are read from the second and successive copies.