So is the external drive connected by USB ? If so, is it connecting to a USB-2 port or a USB-3 port?
You should not require additional drivers on a USB-2 system but if you are running from a USB-3 port then drivers will be required.
Secondly, the largest "standard" partition you can create is 2Tb. To have a 2.5Tb single partition, it needs to be formatted as GPT (GUID Partition Table). For test purposes, you can create a single 2Tb partition and leave the rest pending further testing.
You don't mention a make for the external drive you are using, but for backup purposes cheap is never a good choice.