Yes - live booting via removable media can enable the adversary access (read-only or read-write, depending on drivers) to the local HD. Without going into too many details, an OS can only provide protection while it's loaded. So if you bypass the OS, then the data is not protected.
With that said, you can protect against this type of situation by implementing physical controls. You can lock down the BIOS/firmware on a PC to disable boot from removable media and network (PXE boot). However, if the adversary has physical access to the system, they can easily walk away with the HD or perhaps the entire system and pop the HD into another PC.
Alternatively or in conjunction with the above, you can also protect the data through encryption. You can selectively encrypt data or you can use Full Disk Encryption. FDE encrypts all data on the HD with the exception of a small boot loader. FDE makes it much more difficult for an adversary to access the data, even when they have physical access to the PC. The only way the adversary can decrypt the data is by entering the correct authentication token (i.e. password, USB key, smart card, etc) or in rare instances, weaknesses in the FDE implementation (but this is extremely rare).