It's updating the boot sector because you installed a new bootable OS - there was no other way it was going to be able to start.
When you install an operating system, it always touches the boot sector of the bootable media no matter where you install it - every OS has done this.
As for how to recover, no you're not going to be able to without help from your IT department as Windows 8 has removed the EEPC boot loader.
Got it, thanks. In that case, how could I dual boot? Is it even possible?
What if I disconnected the internal drive, installed Windows 8 on the external drive, then reconnected the internal? Would that leave my machine in a state where both drives were bootable but unaware of each other?
yes it's possible with the support of your IT team - but it's not trivial to set up. Your machine is encrypted to protect the data on it, by using another OS you're probably breaking your company data protection rules.
If you did as you suggest, you'd be able to boot whichever drive was plugged in. If you had them both plugged in though, most likely the internal drive would boot as it (usually) has a lower ID.