backing up email / saving maildir on external hard drives

Sat Aug 8 04:56:55 UTC 2015

Hello,
Yesterday I tried to back up a 40GB maildir .
I tried to move the maildir from home to external HDD but failed. Decided
then to compress it(which took several hours). Now changing the disk format
from FAT to exFAT to allow the transfer for the large compressed file.

How does one back up emails on a external drive?
Some advice would be greatly appreciated.