Internal SATA hard disk shows up as removable device on Windows 7

This happens if the SATA controller is set to AHCI mode. AHCI supports hot swapping, and because of this, you can ask Windows to "Safely Remove" an internal hard disk, disconnecting it from the OS and replace it without having to power down the system.

Related, to stop the hard disk from showing up as removable on windows XP, the answer from here (modifying the registry), worked for me:

Real removable devices (USB) still show up as normal, but it removes the internal SATA controller drives from being displayed.

For nForce chipsets, you can add the following to registry: In HKEY_LOCAL_MACHINE \ SYSTEM \ CurrentControlSet \ Services \ nvata, add DWORD DisableRemovable, set to 1.

Reboot.

64-bit systems use nvata64. Some chipsets might use nvatabus instead.

This doesn't work with VIA chipsets (driver vide or videX64), the only option I've found for them is to use older drivers that don't support SATA hotplug.