Well, it may not be mkexfatfs's problem...

I tried using gparted to re-format it FAT32 - Everything "completed successfully" but same result on replug, all the files are there.

I tried deleting a directory on it, and "emptied the trash", unplug, replug, directory is back. 

[86799.359224] scsi 10:0:0:0: Direct-Access     Generic  STORAGE DEVICE   1404 PQ: 0 ANSI: 6
[86799.359637] sd 10:0:0:0: Attached scsi generic sg2 type 0
[86799.571164] sd 10:0:0:0: [sdb] 31116288 512-byte logical blocks: (15.9 GB/14.8 GiB)
[86799.573477] sd 10:0:0:0: [sdb] Write Protect is off
[86799.573481] sd 10:0:0:0: [sdb] Mode Sense: 21 00 00 00
[86799.575487] sd 10:0:0:0: [sdb] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[86799.588396]  sdb: sdb1
[86799.593493] sd 10:0:0:0: [sdb] Attached SCSI removable disk
[86799.809090] FAT-fs (sdb1): Volume was not properly unmounted. Some data may be corrupt. Please run fsck.

Also, I've tried clearing the dirty bit and I can keep running fsck and it keeps saying it's dirty but per the dmesg output write protect is off. 

I'm at a loss...

Richard