Some more info:
The directory of choice now lives on a vfat drive, previously on a ntfs drive.
Before opening tb, I mount the relevant drive, previously with
'sudo ntfs-3g /dev/sdb2 /mnt/mail', and currently with
'sudo mount -t vfat /dev/sda2 /mnt/mail'.
The first case worked, while 2nd does not. However, if I use rox and mount /dev/sda2 to /mnt/sda2 (by clicking on it), tb proceeds fine.
The weird thing is, /mnt/mail is available in both cases. It's as if /mnt/mail is not registering properly, so tb thinks it is not mounted.