There is a bug with SharePoint Designer 2007 site backup and restores! If the backup files (*.cmp) are getting split up into more than 2 files, the catalog file is added into the last of the files. If you want to restore with SharePoint Designer it is looking for this catalog file in the first .cmp file. The solution is to extract all files and create one big .cmp file.
1. get cabarc.exe from the internet;
2. rename all .cmp files to .cab and extract all files;
3. run cabarc N name.cab namedir\*.* ;
4. rename name.cab to name.cmp;
You still have to watch out for max. file size and max. upload limit!
Depending on the size of your backup file it might be necessary to increase the limits on the ShrePoint server for the time of the restore.