Do you have enough space on your server to hold 1 Full + 28 incrementals (aside from USBs) ? The idea is to keep 1 Full + 28 daily incremental backups right on your server and utilize a backup copy job, that will copy your incrementals to some remote machine (either linux or windows) fast and wisely, so that you will have an independent backup chain outside of the office. Sounds good?2 drives are necessary to have one on the server and one out of office in case of disaster.
4 drives are used to expand the time of available backups to four weeks.
A maximum of one week old data, out of office, is acceptable by the business.
P.S. USB drives are woefully slow, which causes problems with performance during backups.