If you are merely looking for an offsite copy, try in your local area for a information storage company. One of the things they usually offer is an offsite backup storage facility, where they turn up at your office in the morning, and you give them your tape in a locked box. They then store it in their secure facility. We run a 2 week rotation, which means the guys bring back the tape we need for that nights backup when the pickup the previous nights backup.
As for your branch offices, doing a remote backup like that will most likely prove ineffective. If its to a member of staff from that office, they will run into the same issues I highlighted before. If its going to your partners machine, then that means there are going to be several data streams all competing for the same bandwidth.
This also assumes that you have a healthy upload speed to the internet from the office. If you are using ADSL broadband, or cable, then your upload speed is not large. You would require something like SDSL, or even MPLS/National Ethernet connections (which are rather expensive)
If you are determined to go down the internet backup route, then look at your internet connection in the office - will it support this sort of function? If so, do a couple of test runs. Remember, you'll have to setup a secure link between the office and the home PC, using some sort of VPN software. Also remind your partner, that if he does this, he pretty much blocks his own use of his home pc until the backup is complete, as anything he does at home will slow the process even further. Look at online companies who offer secure storage space, and look at creating a scheduled task to upload to there.
My honest advice would be to look at off-site storage of your existing backup system. Doing it over the internet to a home pc, using an external HDD, is just too time-consuming, never mind being relatively unsecure, and would eb subject to far too many possible problems.
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Matt
Life is all shadows and dust.
Live it up with women and wine while you can