obsdata <disc-name>tells the system to use the disc disc-name, which must be one of the set of discs which UltraDAS is configured to known and accept. The disc name must be an absolute path.
obsdatatells the system to use the emptiest available disc of those configured.
obsdata /obsdata/intgselects the disc intg. Data discs are almost always mounted under the /obsdata branch of the file-system, and their names are usually the name of the associated telescope followed by a lower-case letter.
It is possible for a disc to be configured and mounted but still unavailable: it may be full or it might be broken; it might even be write-locked! In this case, the system will not select the disc under any circumstances.
The selectable "discs" are technically partitions of discs, and it's possible to configure two partitions on the same physical disc as separate choices. However, this would be a daft way to use the kind of discs that ING normally deploys. In practice, a partition always fills the whole disc.
UltraDAS considers a disc to be full when there is too little available space to fit in two more observations.