INSECURE?

Mrk,
Yes, it would be extreme if malware authors had to rely on fdisk to do their damage. I brought up these readily available utilities as an example that the damage could be done even with these. Malware authors have much better tools at their disposal (integrated into the malware) to access the hard drive, reboot or not.
While I am not a programmer, there are methods to access the drive while in windows. Look at what Partition Magic can do. Will malware hijack Partition Magic? Of course not, but the same programming techniques and methods are available to malware authors. I think the programming techniques are the "open for write" and "write data" methods. There are probably several different methods of low-level disk access within windows that could occur.

And even with my limited programming skill, I can think of ways to use fdisk with command line options (with a reboot) and launch it via autoexec.bat. No physical access necessary, just cause the computer to freeze up, and the user reboots, executing the batch and fdisk.
I have not actually tried this with fdisk, I prefer to spend time fixing problems rather than creating them.

Anyway, the other utilities I listed do not need a reboot (not that a reboot is a huge obstacle for malware) and can access Linux partitions from within windows.
It is true that if the data is encrypted then the information within is protected from viewing. Unless the malware also installs a keylogger to gather your password. Encryption does not prevent the area on the hard drive where the encrypted file or partition resides from being overwritten.
Also, malware does not require an internet connection to do damage.

What is Linux EFS?
Where can I learn more about it?