Maxtor one touch iii drive no longer accessible

Hello,
Mine is somewhat different from other query. We have software Xclone that we use for copying hardrive and our MAxtor one touch iii USB, was an external hard drive that was seen in Disk management as G: drive. We use this as image back up of our Windows 2003 database server. Last week I did a Xclone Pro backup with the mode:backup the entire volume by copying incrementally. Then later on when I re-check it the G: drive was gone completely (looks like it was deleted) and missing??? How that this happens? Please advice. Thanks anyone.