The construction of electric motors has many variants based on the size and requirements of the load applied. Certainly the construction for 360 kW, 1000 rpm, with thrust bearing, vertical motor of this thread is far apart from that into a PC disk driver. Hopefully this is not an oil lubricated bearing.
You should disassembly the motor and replace the thrust bearing (if several, one will be replaced) with an equivalent size deep groove ball bearing, this will provide radial stability to the rotor-shaft assembly during the test while the motor is horizontal. Some motors have back to back angular contact bearings on the thrust side, if so you are lucky and the motor will run horizontally with no problem. After test, the original assembly must be restored. Be careful handling bearings since them deteriorate easily.
This is expensive but other way the rotor will rub against the stator with possible permanent damage to the windings and core lamination.