Well actually Removable media is a subset of physical media. Its the old "a cube is always a beam, but a beam is not always a cube." thing.
Boot sector viruses have been know to be spread through:
- Hard Disks
- Floppy Discs
- CD/DVD/Blu-ray Discs
- BIOS and Secondary BIOS infections
- PCI equipment (I only know of lab-experiments like this, but they do prove its possible)
- Firewire devices.
- USB devices.
- Downloads(quite rare there actually)
For most cases the way the work is the look for the Primary MBR (Master Boot Record) and modify its content with there own payload so the Virus is loaded on each boot. It usually have some companion app that it will retrieve as soon as possible to do the reinfection on different devices and redistribute the virus to other devices. so while on themselves they often do not pose a risk for removable media. They are hard to get rid of. they are more a thing of the past though, since secure boot and the UEFI environment its harder to implement scheme's like this undetected.