Added on external

I have a my cloud drive that is almost full. I installed a WD my desk external to the cloud drive via the USB. I am getting an error saying the new desk top external is not formatted correctly and the my cloud drive will not read the new external. How or what do I need to do

According to the user manual on Page 106 : https://www.wdc.com/content/dam/wdc/website/downloadable_assets/eng/user_manual/4779-705103.pdf

The WD My Cloud device supports the following formats for externally attached USB drives:
FAT32
NTFS (read and write capability)
HFS+J
Linux Ext2, Ext3/Ext4
XFS

Interestingly it doesn’t say exFAT

Oh, found this … My Cloud: External USB Drive Supported File Systems

No exFAT support on the WD My Cloud (only EX Ultra, EX/DL/PR2100/4100 series)

Thank you for the Info. I currently have it attached to a PC. If I format it different would I still be able to see it on a Mac?