Hi, To begin with, I am using a WD (Western Digital?) external hard drive that I back up to wirelessly through an Apple wireless router. The hard drive is divided into three sections - one for each of two iMacs and one for my laptop. The iMacs back up automatically, but I manually start Time Machine for my laptop since it is not always on and not always in my home. I am having problems only with the laptop backing up - both iMacs back up to the HD just fine. Also, if it matters here, my laptop's internal HD recently crashed and I got it replaced at the Apple store, and restored it from a backup that did work two weeks ago. Basically the issue is that I try to initiate Time Machine now with the proper partition opened on the external HD and it reaches 99% after a slow process of "Calculating Changes" and freezes there. This is true wirelessly and if I jack the laptop into the external HD directly with the cord. After several minutes, the fan starts up. After what feels like a pretty long time, it suddenly starts over in "Calculating Changes", reaches 99%, and once again freezes. Does anyone have any idea what the problem could be? I am hesitant to blame it on getting a new HD, but that IS the only major thing that has changed since I last backed up. I am pretty sure it is a software issue since both iMacs back up with no problems. Thank you for any advice you can give!