Backup a bunch of TB

Hi everybody,

I've been struggling with this decision for a long time now and I thought perhaps the most knowledgeable ones here could help me.

I currently have about 5TB of data in a RAID5 server. Most of the information is required to be online due to the randomness of the usage it's given, and therefore most of it gets changed frequently.

To perform backup, I have set up an almost identical server next to it, and sincronization (yes, it's not backup per-se) is performed weekly over FTP. On top of this, an external 1TB USB hard drive is used to perform daily incremental backups.

The external HD is swapped and taken off-site frequently, but the mirror server IS NOT, since it's quite hard to take away. Because of this I have no real off-site backup, and of course this is much of a problem.

I would like to know what would you recommend under this scenario, so that a backup could be taken offsite with ease. Availability is not a major problem so that I don't need to have an extra copy onsite (the server is already running RAID5) if an offsite copy is up to date.

I've thought of an LTO-4 solution, but I have a newbie question: would the software backup take care of spreading the data to the different number of tapes required (800GB/tape | 6TB of data -> about 8 tapes needed)? If this is the case, could a differential backup still be performed to keep more than one copy of data?

Any help will be greatly appreciated as I'm a little lost on this subject, and quite worried.

Thanks in advance. Best regards,
Matias