Mounted volumes can disappear from the left panel of the ARCHICAD file dialog (e.g. open file dialog, add library dialog, hotlink dialog). The problem crops up when the file dialog is brought up again after browsing into a folder of the mounted drive. The roots of this issue lie within the changes Apple made in the mounted volume handling for OS X Yosemite and it is not specific to ARCHICAD.
The mounted drive is still accessible via the Devices section. (Devices section can be turned on in Finder Preferences by ticking the checkbox next to the local drive.)