It means the backup partition on the second hard drive is full. Also, the load means that the server is having some sort of intensive application/program running which is eating cpu and or memory.
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Actually, if there are two physical CPUs in the server and they are using hyperthreading then a server load of 3.79 is not bad. Basically as a general rule of thumb divide the server load by the number of CPUs. (For a two physical server with hyperthreading (which splits the physical CPUs into two more units) a load of 4 would be approximately the same as a load of 1 on a one processor server. 4 divided by 4 = 1. In this case 3.7 divided by 4 < 4, which is less than 1.
Actually the filled partition of /backup should not affect your server--only the backups. This means that no new backups will be written to the partition. Whether the host is actually putting backups on that partition is another matter. You can put anything there.