If you want to mount a new HDD directly to /home/USER/imap you should have so many partitions on it as users you have there and probably forget about correct disk quotas.
so it would be:
/dev/sdb1 mount to /home/user1/imap
/dev/sdb2 mount to /home/user2/imap
/dev/sdb3 mount to /home/user3/imap
...
...
...
/dev/sdbN mount to /home/userN/imap
I wouldn't do that, this way it is difficult to maintain it unless use LVM.
So my point is to mount /dev/sdb1 (1Tb partition) to any place you want outside the /home, why not to /var/spool/virtual ?
Then you create there the following structure:
/var/spool/virtual/user1/imap
/var/spool/virtual/user2/imap
/var/spool/virtual/user3/imap
...
...
...
/var/spool/virtual/userN/imap
And depending on what way you want to go, either use
mount --bind or modify exim.conf/dovecot.conf to work with imap folder located in /var/spool/virtual/.
This should not be totally correct, since if you mount the partition with quotausr and quotagrp DA should be able to count them too since it should use quota command to count data amount, am i wrong?
That's correct only partly. Directadmin will probably show quotas correct, but users will be able to overpass quota limit. If you have emails and a homedir on one and the same partition and a quota limit of 1Gb, you wont be able to overcome it much.
If you have imap folder on another partition how much quota you will set for an user there? Another 1Gb? Part of it? 100Mb? No limit at all? That of course depends on a hosting packages, so let them decide.