Time Machine Backup to Netgear router attached USB

I have a Netgear Router to which I can attach a USB drive. I would like to have this attached and available all the time for my Time Machine backup.

- I have the instructions from Netgear, which say the drive should be formatted as GUID or MBR

- I have a previously partitioned drive which I've been using for a few months connected directly to my MacBook Pro

- This 500GB drive was partitioned 400GB as the Mac OS Extended format and 100GB as FAT

- I use the 400GB for the Time Machine Backup

- When I plug the drive into the Netgear router it sees the FAT partition but not the other one.

1. Is there a way to change the 400GB partition to GUID or MBR?

2, If I have to reformat the whole drive to GUID or MBR, will either of those formats be bad for future retrieval of Time Machine backup and file permissioning on a new Mac?

Thank you!