Can I replace a power adapter of 1.5A with one of 1A?
No, a 1A power supply isn't a replacement for a 1.5A power supply.  It might work if the disk actually takes less than 1A and the power supply was sized with some margin.  However, these high volume items are are usually trimmed to the bone, so probably not.

You can use a 12V supply with a higher current rating, but not a lower one.  The device will only draw the current it wants, but the supply has to able to put out the maximum current the device wants.  Since that may be in the 1-1.5 Amp range, the 1A supply is not good enough.

The 1A supply probably won't cause any damage.  If the device tries to draw more current than it can provide, the voltage will collapse or it will deliberately shut down.  Shutting down clearly can't hurt the device.  Having a partway voltage for extended periods of time could possibly be bad, but that's rather unlikely.  Of course the device won't work right with the partial voltage.  It might not tell you about it and just mess up occasionally.