Unable to mount FAT16 partition (Invalid Argument)

[Peter Erickson]

I'm hoping to get some assistance mounting a FAT (FAT16?) partition 
created by the VMware ESXi installer (version 6.7.0). Every attempt to 
mount it on 11.2-RELEASE-p4, results in an "Invalid argument" error, 
without much to indicate what is wrong or how to resolve it. Any advice 
or direction would be much appreciated.

# gpart show ada1
=>      34  16777149  ada1  GPT  (8.0G)
         34        30        - free -  (15K)
         64      8128     1  efi  (4.0M)
       8192        32        - free -  (16K)
       8224    511968     5  ms-basic-data  (250M)
     520192        32        - free -  (16K)
     520224    511968     6  ms-basic-data  (250M)
    1032192        32        - free -  (16K)
    1032224    225248     7  vmware-vmkdiag  (110M)
    1257472        32        - free -  (16K)
    1257504    585696     8  ms-basic-data  (286M)
    1843200   8386560     2  ms-basic-data  (4.0G)
   10229760   6547423     3  vmware-vmfs  (3.1G)

# ls /dev/ada1*
/dev/ada1        /dev/ada1p2     /dev/ada1p5     /dev/ada1p7
/dev/ada1p1      /dev/ada1p3     /dev/ada1p6     /dev/ada1p8

# file -s /dev/ada1p5
/dev/ada1p5: DOS/MBR boot sector, code offset 0x3c+2, OEM-ID "MSDOS5.0", 
sectors/cluster 8, reserved sectors 2, root entries 512, Media 
descriptor 0xf8, sectors/FAT 250, sectors/track 32, heads 64, sectors 
511968 (volumes > 32 MB), serial number 0x4e781016, label: " 
", FAT (16 bit)

# mount -t msdosfs /dev/ada1p5 /mnt
mount_msdosfs: /dev/ada1p5: Invalid argument

While this isn't the drive I initially had the problem with, I have been 
able to recreate it using VirtualBox.

# dmesg | grep ada1
ada1 at ata0 bus 0 scbus0 target 1 lun 0
ada1: <VBOX HARDDISK 1.0> ATA-6 device
ada1: Serial Number VB039c38ca-974cc97c
ada1: 33.300MB/s transfers (UDMA2, PIO 65536bytes)
ada1: 8192MB (16777216 512 byte sectors)

FWIW, knoppix has no issues mounting the partition, but I'd really be 
interested in getting this to work on FreeBSD.

knoppix # mount | grep /dev/sdb5
/dev/sdb5 on /mnt type vfat 
(rw,relatime,fmask=0022,dmask=0022,codepage=850,iocharset=utf8,shortname=mixed,errors=remount-ro)

Based on the info from knoppix, I tried to mount the partition under 
FreeBSD using mount_msdosfs directly and specifying the locale 
(en_US.UTF-8) and codepage (CP850) , but the result was the same - 
"Invalid argument."

Thanks in advance.