I think it will help if you concentrate to solve one problem at a time.

Say for the 750 Maxtor USB hard disk. Can you try it on other operating system? If there is a hardware fault and the disk cannot be read/write properly the operating system could abandon the attempt. There is also a possibility the update isn't perfect and there are bug in the installed Linux.

When you mount a partition have you tried to mount it as root manually with a terminal? The server grade version of Debian could have additional security to prevent users to mount partitions one doesn't owned.