-how can the USB drives be umounted/mounted automatically when the person on site changes it (monday to tuesday, for example). There will always be only one HDD conected at the time.

By a Stardome Enclosure, and at leas 3 disks. Then you always has the most current backup online in the raid - and the user just change disk for offsite storage. The enclosure is also much better build than normal singel disk usb enclosure -

http://www.stardom.com.tw/sohoraid%20sr3610.htm "SR3610 series bundle with a hardware controller internally to provide with not only RAID 1 function for instant data"

-- hs