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Date: Wed, 19 Oct 2016 03:44:19 +1100 (EST)
From: Ian Smith <smithi@nimnet.asn.au>
To: doug <doug@fledge.watson.org>
cc: freebsd-questions@freebsd.org
Subject: Re: Can not mount USB device
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In freebsd-questions Digest, Vol 646, Issue 2, Message: 14
On Mon, 17 Oct 2016 16:25:36 -0400 (EDT) doug <doug@fledge.watson.org> wrote:

 > The system is FreeBSD 10.3-RELEASE-p7 #0. All commands are execured as root.
 > 
 > file -s /dev/da0s1
 > /dev/da0s1: DOS/MBR boot sector, code offset 0x58+2, OEM-ID "MSDOS5.0", 
 > sectors/cluster 8, reserved sectors 38, Media descriptor 0xf8, sectors/track 63, 
 > heads 16, hidden sectors 8192, sectors 831488 (volumes > 32 MB) , FAT (32 bit), 
 > sectors/FAT 813, serial number 0x482393a9, unlabeled
 > 
 > I ended up with the following attempts
 > 
 > mount -t msdosfs -o -m=644 /dev/da0s1 /mnt/
 > 5mount_msdosfs: /dev/da0s1: Permission denied
 > 
 > dmesg:
 > (da0:umass-sim0:0:0:0): Error 13, Unretryable error
 > g_vfs_done():da0s1[WRITE(offset=19456, length=4096)]error = 13
 > (da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 00 00 20 26 00 00 08 00
 > (da0:umass-sim0:0:0:0): CAM status: SCSI Status Error
 > (da0:umass-sim0:0:0:0): SCSI status: Check Condition
 > (da0:umass-sim0:0:0:0): SCSI sense: DATA PROTECT asc:27,0 (Write protected)

Looks like it either has a write-protect switch, as some do, or it came 
write protected, ie is designed to be read-only, as apparently some are.

 > mount -t msdosfs -o -m=644,-M=644 /dev/da0s1 /mnt/
 > mount_msdosfs: /dev/da0s1: Permission denied
 > 
 > dmesg:
 > (da0:umass-sim0:0:0:0): Error 13, Unretryable error
 > g_vfs_done():da0s1[WRITE(offset=19456, length=4096)]error = 13
 > 
 > All other variations got the same error (Permission denied)
 > 
 > The USB stick is readable in windows 10. I thought it was a data disk it turns 
 > out to have 2 install program files one for windows and one for mac.

So try mounting it read-only.  As root, you don't need to mess with the 
permissions either, so just this should work:

 # mount -t msdosfs -r /dev/da0s1 /mnt	# alternatively '-o ro'

cheers, Ian