OK it was a volume bitmap issue with the front card again - this seems to happen in camera so for now even though Sandisk Ultra works as exFAT I've got to say it's too unreliable.
Code:
C:\Users\reverend>chkdsk h:
The type of the file system is exFAT.
Volume Serial Number is 3A0C-1E0E
Windows is verifying files and folders...
Volume label is HD2_F.
Windows found errors on the disk, but will not fix them
because disk checking was run without the /F (fix) parameter.
Corruption was found while examining the volume bitmap.
File and folder verification is complete.
Windows has checked the file system and found problems.
Run CHKDSK with the /F (fix) option to correct these.
62334976 KB total disk space.
35108096 KB in 179 files.
640 KB in 5 indexes.
0 KB in bad sectors.
256 KB in use by the system.
27225984 KB available on disk.
131072 bytes in each allocation unit.
486992 total allocation units on disk.
212703 allocation units available on disk.
C:\Users\reverend>chkdsk h: /f
The type of the file system is exFAT.
Volume Serial Number is 3A0C-1E0E
Windows is verifying files and folders...
Volume label is HD2_F.
Corruption was found while examining the volume bitmap.
File and folder verification is complete.
Windows has made corrections to the file system.
No further action is required.
62334976 KB total disk space.
35154176 KB in 180 files.
768 KB in 6 indexes.
0 KB in bad sectors.
256 KB in use by the system.
27179776 KB available on disk.
131072 bytes in each allocation unit.
486992 total allocation units on disk.
212342 allocation units available on disk.
C:\Users\reverend>chkdsk h: /f
The type of the file system is exFAT.
Volume Serial Number is 3A0C-1E0E
Windows is verifying files and folders...
Volume label is HD2_F.
File and folder verification is complete.
Windows has scanned the file system and found no problems.
No further action is required.
62334976 KB total disk space.
35154176 KB in 180 files.
768 KB in 6 indexes.
0 KB in bad sectors.
256 KB in use by the system.
27179776 KB available on disk.
131072 bytes in each allocation unit.
486992 total allocation units on disk.
212342 allocation units available on disk.
C:\Users\reverend>
Basically when the volume bitmap issue arises its head is when the camera starts doing funky things like stopping recording, hanging on shutdown and so on. On this occurrence it looks like it happened because the camera started then shutoff quickly when the car stalled.
This is the sort of thing I think Sungmoon means when he says Sandisk can have issues. Maybe they're using new controllers in the 128GB cards which is why
@Dionic90 is not having the issues.
I'm not sure if it's possible to have the camera scan the filesystem at startup using the equivalent of fsck / exfatfsck / chkdsk - if that was an option it would wipe out any problems like this as it would be self healing?