I configured some iSCSI targets to connect to from an Windows Server 2012R2 and everything went pretty good. Then, the two connected virtual disks disappeared from the windows server, even tho the iSCSI initiator showed them as connected. They were missing in the device manager and disk management screens. Restarting the windows iSCSI initiator service as well as the tgt-daemon on the linux side did not help. Logfiles did not indicate any issues.
[Solution]
While the first disk was configured by hand in an config file in /etc/tgt/conf.d/ I configured the second one using Webmin. Webmin writes the configuration into /etc/tgt/targets.conf instead of a single snopped in the conf.d/ subfolder.
I cut/pasted the second targets configuration from targets.conf to it's own config file residing in /etc/tgt/conf.d and restarted the tgt-daemon.
After the next reconnect from the windows side, the devices connected successfully and the missing virtual disks reappeared.
It looks like tgtd does not like "mixed configs" in both targets.conf and snippets in the conf.d/ subdir
