MyDefrag is totally safe and absolutely cannot make a disk invisible like that. It does not even access the partition information. Your problem is definitely caused by something else on your computer. Some softwares are incompatible with defraggers in general, see the second point on Known problems
. Are you using software like that?
Could it be a permission problem? Does the userid that you are logged in on have enough permissions to access the disk? Please note that logging in on the administrator userid does NOT necessarily give you enough permissions.
Also, I remember having a similar problem a long time ago. It was caused by scumware, just visiting a website was enough, it installed itself and did all kinds of nasty things, including making disks invisible. I don't remember the solution, something to do with a registry setting. It was Windows 2000, I think, definitely not Windows 7.