Indeed, this is a bug in 1.4 (Windows version only). The application cannot retrieve the available space on Windows when the swap files directory is a sub-directory (non-root) of a removable device.
The work around is just to change the swap directory to a root directory on the removable disk, or much better, to a directory on an internal hard disk (non-removable):
- Edit > Global Preferences
- Select Directories and Network on the left panel.
- Remove the current directory from the Swap storage directories list.
- Add a new directory that simply points to the removable device, without any directory specification beyond the root. For example, F: or F:\, but not F:\my_temp_dir\pixinsight.
This will be fixed on 1.4.5.