I run a Time Machine backup every weekend to an external USB drive. Usually the transfer rate is at least about 40 - 50 MB or higher. I did a backup today, before installing the 10.7.3 update and the transfer rate fluctuated from about 1MB to say 20MB. Much slower than normal. After installing the update I ran another backup, and it's transfer rate is around 1MB per second. Is there any way of finding out why it's gone so slow?