Why are hard drives mounted by default in Windows OS, while they aren't in Linux OS? Is that a security issue?
If your computer was hacked, can unmounted drives be considered clean?
Typical user-oriented GNU/Linux distros have automounters on by default.
More like a usability issue.
No, because the attacker could mount them, do his dirty stuff and unmount them again, and wipe the logs. Only careful inspection of FS metadata will reveal when it was mounted (provided that the attacker didn't mess with it, which you likely won't be able to prove).