Why my USB disk drive is mounted "WriteProtect is on"?

Why my USB disk drive is mounted "WriteProtect is on"?

[Peter Teoh]

Got no reply from linux-usb.   So trying here.   In the past I was
able to mount it as "WriteProtect is off"....but one day....suddenly
it became "on" on two of my machine, but the others like 3 still
"off".   So right now the status is as follows:

usb-storage: waiting for device to settle before scanning
usbcore: registered new interface driver usb-storage
USB Mass Storage support registered.
scsi 6:0:0:0: Direct-Access     WD       1600BEAExternal  1.02 PQ: 0 ANSI: 0
sd 6:0:0:0: [sdc] 312581808 512-byte hardware sectors (160042 MB)
sd 6:0:0:0: [sdc] Write Protect is on
sd 6:0:0:0: [sdc] Mode Sense: 12 a1 9e af
sd 6:0:0:0: [sdc] Assuming drive cache: write through
sd 6:0:0:0: [sdc] 312581808 512-byte hardware sectors (160042 MB)
sd 6:0:0:0: [sdc] Write Protect is on
sd 6:0:0:0: [sdc] Mode Sense: 12 a1 9e af
sd 6:0:0:0: [sdc] Assuming drive cache: write through
 sdc: sdc1 sdc2 sdc3 < sdc5 > sdc4
sd 6:0:0:0: [sdc] Attached SCSI disk
sd 6:0:0:0: Attached scsi generic sg2 type 0
usb-storage: device scan complete

But when u plug out and insert into another machine, it may be
detected as "off", why?

With this writeprotect on, there is no way i can mount it for writing.
  Appreciate your answer very much.

THanks.

-- 
Regards,
Peter Teoh