If your eSATA drive is not detected when you plug it in, there are some options to solve the issue without reboot with the eSATA plugged in.
# echo 0 0 0 | tee /sys/class/scsi_host/host*/scan
[Detect new eSATA drives - ArchWiki].
2. Mount disk
1. Unmount disk(s)
2. Force drive to enter the standby mode
# hdparm -y /dev/sdb
3a. Power drive off, or
3b. If you want to spin drive up again
# sg_sat_set_features -f 7 /dev/sdb
Power-up HDD in standby - Arch Linux Forums
HDD Power-Up in Standby - Super User