The problem you are having is due to the largest file that you can address having to be smaller than the available free space on your original WHS hard drive.
This is why the WHS documentation recommends a minimum 80 GB primary drive - what is not clearer, is as soon afterwards, you should add additional drive space "before" moving large files over to the WHS. Similarly, you should begin your build of your WHS with as large a primary HDD as possible and it is recommended that you add more HDD space as soon after your initial build as possible - to prevent many backups form taking up all the space on the WHS.
I would remove all backups you have stored then remove and re-add your additional drives and allow everything to settle down and then move your files back onto the WHS - making sure that in any single file transfer, the largest file(s) in the copy do not exceed the available capacity of your primary WHS drive/partition as at DATA (D:) in Computer | Manage | Disk Management
The word to watch here is, "start off your WHS server build with as large a single drive as possible - then add your additional drives and then begin your file copies ot the WHS - by doing this you'll have far fewer problems and have plenty of space to support the copy of larger files.
