Latitude E6540 - Bitlocker keeps asking for password

I have a Latitude E6540, running Windows 10 Enterprise (Anniversary Edition - 1607)

The main drive (256GB SSD) has been encrypted with bitlocker and that's working correctly. However I also have a USB flash drive (Kingston Data Traveller 3, 8GB) which is also encrypted by bitlocker.

The problem is that despite entering the password to unlock the drive, after a few minutes it locks the drive and asks for the password again. This is very, very, very annoying. Since it interrupts whatever I'm doing - Explorer shuts down, Office is unable to save files etc.

I also have several external USB hard drives (a mix of USB2 and 3) and they act the same way.

It only happens on this laptop, if I put the USB drive in my home PC (also Windows 10 Anniversary 1607 but Professional Edition not Enterprise) this does NOT happen.

As you can guess this getting more than a little annoying.

My best guess at this is that the USB controller is "dropping" the drive and then rediscovering it. With an unencrypted device this doesn't matter as it redetects and re-mounts the drive. With a bitlocker (or other encrypted) drive it has to ask for the password again. (and again and again and again ... you get the idea!)

I've tried turning off the "allow windows to turn off this device to save power" on the USB controller but that has had no effect.

Any ideas what is happening and how it can be fixed?