I recently bought an Iomega NAS drive but I want to install my own operating system on it. I connected the hard drive to my computer with a SATA to USB casing but under disk management it says it is a GPT Protective Partition. How can I remove the partition and install my own OS while still being able to run in the Iomega NAS casing?
I don't know about NAS-specific issues, but if the disk has no data you can use most MBR partitioning tools to do the job. Since you mention "8" (presumably Windows 8), you can see Microsoft's official documentation on the topic, for example.
If the disk already holds data, you can use GPT fdisk (gdisk) to convert GPT to MBR while preserving your existing partitions.
Note that disks bigger than 2TiB pretty much require GPT; MBR maxes out at 2^32 sectors, which works out to 2TiB, given 512-byte sectors.