I'm using virtual box to boot my old laptop's Windows 10 Hard disk drive into a VM. Its a bit slow so I was wondering if there was any concern installing the Virtual box guest additions software onto the hard disk. I've already backed up the data on the HDD but just want to understand the possibility of corrupting data on the HDD before making any changes like these. I would like to be able to preserve the HDD once I'm done editing in the VM environment.
No, this is safe to do. The VirtualBox Guest Addons is a library that runs as software. It provides a connection between the guest and host to increase performance. If you don't need it anymore, you can uninstall the guest addons.
It may be required to do so before you use the harddisk in the old laptop, but even then it should not cause any problems.