error using @volume@ to substitute for drive letter

I have Toucan on a USB drive (label USB1TB), and a working backup job for the My Documents folder.

I want to make the job flexible using variables so it's not dependant on the PC its used on and other peripherals etc.

When target drive is: F:\Backups\LaptopMyDocs the job works

I changed this to: @volume@\Backups\LaptopMyDocs and hit Preview.

The displayed drive changes to: USB1TB\Backups\LaptopMyDocs, but on Preview all files and folders are blue (= will be copied) which is incorrect - they are already on the drive.

If I run the job all copies fail with "(error3: the system cannot find the path specified)"

I inserted a colon: @volume@:\Backups\LaptopMyDocs. This also fails,- but does not even find the drive.

Can someone explain what i'm missing please?

Thanks!