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Hieu,

Sorry if this is a bit pedantic (I'd rather give you the tools than write the code to give you your exact answer).

Start with the VolumeToBSDNode sample (developer.apple.com).  That doesn't quite do what you want but that's your first step.

Now, plug your USB storage device into your machine.  When it mounts, open a Terminal window and execute the "df" command

e.g.

$df

Filesystem                  512-blocks      Used      Avail Capacity  Mounted on
/dev/disk0s2                 487725344 322404632  164808712    66%    /
/dev/disk4s2                   2212616    702824    1509792    32%    /Volumes/iDisk
/dev/disk5s1                   2004912   1605848     399064    80%    /Volumes/Key

disk0s2 (disk 0, slice 2) is my root device
disk4s2 is where my iDisk mounts.  (disk4s2 is the bsd name)
disk5s1 is my USB Key

Find the bsd name that corresponds to you USB Key
now type in "ioreg -l" and you'll get a rather large dump in your Terminal window.

Find the bsd name that corresponds to your usb key (e.g. disk5s1)

Mine is:

    | |   |     | |             +-o Apple_HFS_Untitled_1@1  <class IOMedia, registered, matched, active, busy 0, retain count 8>
    | |   |     | |               | {
    | |   |     | |               |   "Leaf" = Yes
    | |   |     | |               |   "Writable" = Yes
    | |   |     | |               |   "BSD Minor" = 17
    | |   |     | |               |   "IOBusyInterest" = "IOCommand is not serializable"
    | |   |     | |               |   "Partition ID" = 1
    | |   |     | |               |   "Preferred Block Size" = 512
    | |   |     | |               |   "UUID" = "DD42F217-92C7-416C-84A6-E0296AE9BDA7"
    | |   |     | |               |   "BSD Major" = 14
    | |   |     | |               |   "BSD Name" = "disk5s1"
    | |   |     | |               |   "Size" = 1026514944
    | |   |     | |               |   "Content Hint" = "48465300-0000-11AA-AA11-00306543ECAC"
    | |   |     | |               |   "Removable" = Yes
    | |   |     | |               |   "BSD Unit" = 5
    | |   |     | |               |   "Ejectable" = Yes
    | |   |     | |               |   "Content" = "48465300-0000-11AA-AA11-00306543ECAC"
    | |   |     | |               |   "Whole" = No
    | |   |     | |               | }
    | |   |     | |               |


Scroll up and you will see an IOMediaBSDClient, an IOMedia (where "Whole" = Yes), ... and further up the food chain you will see an IOUSBMassStorageClass and then then IOUSBInterface (etc., etc).   Some people prefer to use IORegistryExplorer (/Developer/Applications/...).

Now do the same for the bsd name that corresponds to your root device.  Notice any differences that you could search for?  :-)

Now, from user space each of these entries (like the one I pasted in, above) are called "io_registry_entry_t", and going up the food chain corresponds to calling IORegistryEntryGetParentIterator() (couple that with IOIteratorNext(), IOObjectConformsTo() and IOObjectRelease() and there's your more or less complete tool set).

VolumeToBSDNode is a nice little project to play around in.

hth,
Cheers,
-H.