In text install, when adding a partition, if the allowable drives selection is changed (eg removing hdb), then changing any other parameters on the page before pressing 'ok' reselects the drive(s), ie it has to be the last option changed.
This issue has been assigned to a developer for further action.
Confirmed; cycling through the "mount point" field will cause the "allowable drives" entries to be reset to all allowable.
Fixed in the Pinstripe beta.