I am using an encrypted hard disk for backing up my (also encrypted) laptop disk, running Ubuntu 15.10 64 bit. The built-in laptop disk works fine but the external USB disk, after working for somewhere between a few minutes and a few hours, suddenly switches itself back to being encrypted and shows the password prompt. This happens even though I keep choosing "remember forever". Often, when I then re-enter the password, either it does not work and the dialog is shown again, or the system claims that it cannot mount the drive because it is already mounted.
I thought maybe this is the problem with the physical disk and I attached a second encryped external usb drive. Now I am seeing the same problems for both disks, of completely different make and model. Although I have seen this problem with 15.04 my impression is that is has become even worse.
The bottom line is that now both my external USB hard-disks are essentially unusable and the whole situation is extremely annoying.
Is this a known issue, and more importanly, is there anything that can be done to fix or avoid these problems?
The error message when trying to unlock again and the system claims something is already mounted is:
Error unlocking /dev/sdd1: Command-line `cryptsetup luksOpen "/dev/sdd1" "luks-xxxxxx-xxx-xxx-xxx-xxxxxxxxxx" ' exited with non-zero exit status 5: Device luks-xxxxxx-xxx-xxx-xxx-xxxxxxxxxx already exists. (udisks-error-quark, 0)