it's iDisk being full of suck. iDisk is really slow, no matter how fast your connection, it only uploads so fast to the server.
For large files, it is possible that your conectino is timing out due to the slow transfer. I had something similar where I would try to upload a large file and it would never finish successfully. I think it had something to do with if you don't "use" the connection for a while, it drops. I was able to upload the large file finally by perodically browsing through some folders in my iDisk using a new Finder window while it was uploading the large file. This way the connection stayed "active" and the transfer completed.
I know it doesn't make sense but it worked for me. Maybe give it a try?
The real solution though is that there is none, unless Apple decides to improve iDisk. I'm pretty satisfied with MM on the whole for its other features, but the iDisk is sorely lacking. Popular belief is that something may be done about this when that NC data center goes online, we shall see.
